Less Fuel In Tank Leads To Better Mileage?

dfwjim1 · Wed Dec 02, 2015 6:12 pm

I was reading an article a while back in the USA Today about how airlines are saving money by managing the fuel loads
on their aircraft so that they are carrying just the right amount of fuel so that fuel is not being burned to carry excess fuel. Would
this type of thing also work when it comes to cars and trucks? For example I drive about 150 miles a week (mostly freeway)
and fill up my vehicle every two weeks. Would I get better gas mileage by buying 1/2 a tank of gas once a week versus traveling
around with a full tank for the first week after filling up? For the record there are numerous service stations in my neighborhood
pay cash when buying gasoline so the only inconvenience for me would be the time to takes to stop in and buy the gas.

Thanks for your responses to my unusual question.

zckls04 · Wed Dec 02, 2015 6:20 pm

I was always told that running a car close to empty puts stress on the fuel pump, so you may find the cost of repairing that outweighs any savings. In theory the gas usage will be lower when there's less weight though.

Osubuckeyes · Wed Dec 02, 2015 6:20 pm

I'm guessing that the extra weight carried in car fuel tanks when filling to full is marginal compared to the extra weight an aircraft would carry filling to full.

Not to mention that running cars on lower fuel tanks causes the fuel pump system to wear more quickly in cars.

diverted · Wed Dec 02, 2015 6:20 pm

Any weight savings will lead to efficiency gains. How much a half tank of gas will save is probably pretty trivial in comparison to thousands of lbs on an aircraft.

If your driving habits and routes are pretty consistent you could do a little experiment. Fill your vehicle half full, and see how far it goes, compared to filling it. IE, if it takes 20 gallons, put in 10. When the light comes on, take a look at your odometer. If you've gone farther than 50% of what a full tank gets, you'll have an idea that you are saving fuel. It's a rough experiment, and doesn't factor in things like traffic, winter vs summer gas, etc.

My opinion is that you probably won't notice a difference in mileage, but it's worth a shot.

Aesma · Wed Dec 02, 2015 6:52 pm

300 miles doesn't seem like great range.

When a road vehicle moves, the engine has two main forces working against it, air friction and gravity. Air friction doesn't come into play at slow speeds, to boot. So weight matters. In the last few years manufacturers in countries where gas prices are sensible have made everything to lighten their vehicles, even luxury ones like the Range Rover.

We're talking shaving 100 or 200Kg off cars that weren't that heavy to begin with, but still much more than 20-30 years ago.

For example the Peugeot 308 GTi :

2008 model, 1.6 200hp => 1399Kg

2015 model, 1.6 270hp => 1205Kg

ANITIX87 · Wed Dec 02, 2015 7:03 pm

Quoting Aesma (Reply 4):
300 miles doesn't seem like great range

Depends on the car and where you're driving, obviously.

In my car, if i drive in the city, my range is about 240 miles (385km) on a full tank (15 gal, 57L).

In exclusively highway driving, I've seen as high as 400 miles. (640km)

On most tanks, I end up getting 310 miles (500km) for an average somewhere around 21mpg (11.2 L/100km)

TIS

Flighty · Wed Dec 02, 2015 7:23 pm

It would work much less for cars and trucks. Wheeled vehicles are far more efficient at carrying weight than aircraft. The only thing more efficient than a rubber tired truck is a train with metal wheels (no tire surface deflection, which is lost as heat).

And of course container ships that float on water use almost no energy to move giant weight - superior to trains, but mostly because they are larger, not because floating is better. In fact, my relatives' boat is much less energy efficient than a car of similar size moving at 40 MPH -- a difference of about 10x.

Edit: After thinking some more, displacement ships do beat trains. Train wheels do have friction. But faster ships encounter hydro drag. So it depends on speed.

And, compare how much fuel a 40 ton semi truck burns compared to a 757 doing the same job. Huge difference I'm thinking around 15x. In turn, the weight of fuel carried is much less. So the fuel saving would be a tiny fraction of 1/15 of the fuel a jet saved.

My estimate for that semis truck is 1/225 the savings of a 757 under roughly similar payload. Might save 225 gal on a plane if you're lucky, or 1 gallon on a truck. This is based on 5 MPG truck vs about 3 gallons per mile (?) for a 757.

[Edited 2015-12-02 12:11:18]

Revelation · Wed Dec 02, 2015 7:49 pm

Quoting Flighty (Reply 6):
The only thing more efficient than a rubber tired truck is a train with metal wheels (no tire surface deflection, which is lost as heat).

You learn something new every day!

Someone was asking me the other day why their tires were so hot. My answer was surface friction, but of course the deformation of the tire when it hits the various ruts in the road causes some more heat from within the tire's fibers.

Ahh, the life of a nerd is so rewarding!

tu204 · Wed Dec 02, 2015 8:03 pm

The heavier your vehicle is, the more energy you need to move it. Therefore more fuel consumption.

I doubt you would notice a big change though since we are talking about lightening your car by let's say 50 liters when the car weights 1200kg (underestimating here, I know). That's just a 5% savings in weight, which will transfer to maybe a 3% savings in fuel burn, at best.

Ken777 · Wed Dec 02, 2015 8:04 pm

My father (a mechanical engineer & car nut) always told me to keep at least a quarter tank of gas. That kept any dissolved matter in the gas from solidifying and gumming up the systems. I still try to do that, especially as the gas gauge is getting less accurate on the 13 year old Caddy.

I also fill up more frequently during winter in case we get snow or ice. I like the idea of letting the car idle with the heater on if I get stuck and have to wait for help. You probably don't have that problem in Florida, but when bad weather heads your way it might be wist to keep at least a 3 quarter tank

Flighty · Wed Dec 02, 2015 8:16 pm

Quoting Revelation (Reply 7):
Ahh, the life of a nerd is so rewarding!

That info came from Autoline After Hours, a great insider auto channel coming out of Michigan. Can't say enough about their program. It stays fresh thanks to the genius of host Jon McElroy. Airs every Thursday. There is info there you won't find anywhere else (at least about the car industry).

[Edited 2015-12-02 12:17:24]

Revelation · Wed Dec 02, 2015 9:16 pm

Quoting Flighty (Reply 10):
Autoline After Hours

Nice, I see they also have audio podcasts:

http://www.autoline.tv/podcasts/feeds/afterhours-audio.xml

I'll have to subscribe so I can listen during my commute to work, thanks!

Seems appropriate to listen to it while driving, no?

vikkyvik · Wed Dec 02, 2015 9:26 pm

Quoting Aesma (Reply 4):
When a road vehicle moves, the engine has two main forces working against it, air friction and gravity.

Do you mean air friction and inertia? If you're on a flat road, gravity is pushing at a right angle to your direction of travel.

(granted, gravity will cause some friction in the wheel bearings and perhaps other mechanical joints, but that's going to be pretty minor).

The primary force your engine has to overcome is accelerating the mass of the car (at least, when accelerating, which is when you burn the most fuel anyway).

Kiwirob · Wed Dec 02, 2015 9:41 pm

Quoting ANITIX87 (Reply 5):
On most tanks, I end up getting 310 miles (500km) for an average somewhere around 21mpg (11.2 L/100km)

That's pretty piss poor mileage, I'd be buying a new car if either of mine had mileage like that.

I had an Indian colleague many years ago who swore black and blue that using his headlights increased his fuel consumption, maybe it did but I wouldn't be running that risk.

dfwjim1 · Wed Dec 02, 2015 9:57 pm

Quoting kiwirob (Reply 13):

My car (a 2005 PT Cruiser) gets between 21 and 24 MPG but it is totally paid for, runs well and I drive only around 15,000 KM
a year so I am going to keep it rather than get a newer vehicle with better fuel efficiency.

vikkyvik · Wed Dec 02, 2015 9:59 pm

Quoting kiwirob (Reply 13):
I had an Indian colleague many years ago who swore black and blue that using his headlights increased his fuel consumption, maybe it did but I wouldn't be running that risk.

Technically it will increase your fuel consumption, I'd think. But probably not noticeable unless you measure over a long period of time.

tu204 · Wed Dec 02, 2015 10:21 pm

Quoting vikkyvik (Reply 15):
Technically it will increase your fuel consumption, I'd think. But probably not noticeable unless you measure over a long period of time.

Of course it will increase fuel consumption, the fact that your headlight are on means you are draining 50-100W of electrical power. This power comes from where? You generator. Which runs off of what? Your internal combustion engine.

Same as driving with your air conditioner on or off.

But as you pointed out, this increase will be miniscule. My advice would be gradual accelerations. This would give you the most noticable fuel savings. To get an object to move from a standstill takes quite some power. Avoid unecessary braking followed by rapid accelerations.

Flighty · Wed Dec 02, 2015 10:28 pm

Quoting tu204 (Reply 16):
Avoid unecessary braking followed by rapid accelerations.

Speaking of heat, my grandfather taught me that using your brakes is just converting gasoline that you paid for into waste heat. Every time you use the brakes on level ground, you are spending money. So, don't race to a red light and hit the brakes for example.

Quoting Ken777 (Reply 9):
I also fill up more frequently during winter in case we get snow or ice. I like the idea of letting the car idle with the heater on if I get stuck and have to wait for help.

Maybe in FL a hurricane escape is an idea? In cold weather states like say North Dakota, you need extra fuel to tide you over when a 2-3 day cold front hits. In particular, you don't want to be on 'E' standing outside filling up when the wind is blowing and it's 20 below or something.

[Edited 2015-12-02 14:32:19]

ANITIX87 · Wed Dec 02, 2015 11:09 pm

Quoting kiwirob (Reply 13):
That's pretty piss poor mileage, I'd be buying a new car if either of mine had mileage like that.

I bought my car for performance. I knew what the gas mileage would be when I bought it. A tank of 16MPG is not unusual for me when I'm driving it hard (pretty often...let's be honest).

TIS

zckls04 · Wed Dec 02, 2015 11:24 pm

Quoting kiwirob (Reply 13):
I had an Indian colleague many years ago who swore black and blue that using his headlights increased his fuel consumption, maybe it did but I wouldn't be running that risk.

That's a pretty common belief in some parts of the world. Driving at night in India is truly terrifying.

Aesma · Wed Dec 02, 2015 11:58 pm

Quoting Revelation (Reply 7):
Someone was asking me the other day why their tires were so hot. My answer was surface friction

They could also be under-inflated.

Quoting vikkyvik (Reply 12):
Do you mean air friction and inertia? If you're on a flat road, gravity is pushing at a right angle to your direction of travel.

(granted, gravity will cause some friction in the wheel bearings and perhaps other mechanical joints, but that's going to be pretty minor).

The primary force your engine has to overcome is accelerating the mass of the car (at least, when accelerating, which is when you burn the most fuel anyway).

Yeah you're right, the famous mix up between mass and weight.

Stealthz · Thu Dec 03, 2015 12:14 am

Quoting vikkyvik (Reply 15):
Technically it will increase your fuel consumption,

Correct, 2x55w Low beam halogen bulbs = 110w. Approximately 1/7 HP

That energy has to come from somewhere, ultimately the fuel tank.

Could you measure the difference?
Not in any meaningful real world way.

vikkyvik · Thu Dec 03, 2015 12:24 am

Quoting tu204 (Reply 16):
Same as driving with your air conditioner on or off.

I hate driving with the air conditioner on. In a manual transmission, the loss of power is very noticeable. Plus I have to adjust shifting time, as the engine spools down quicker.

Quoting tu204 (Reply 16):
To get an object to move from a standstill takes quite some power. Avoid unecessary braking followed by rapid accelerations.

Absolutely. And avoid coming to complete stops whenever you can.

Back when I had a Honda Civic, I did that kind of stuff for a few months - keeping the engine revs low (manual transmission), accelerating gently, keeping the speed a bit lower than normal, avoiding complete stops, keeping the engine in gear while decelerating (I use engine braking anyway), etc.

It's terribly boring driving that way, but I did see about a 3 mpg increase (my driving was mostly city driving).

Quoting Aesma (Reply 20):
Yeah you're right, the famous mix up between mass and weight.

It's all about the slugs.

MSPNWA · Thu Dec 03, 2015 12:39 am

Quoting dfwjim1 (Thread starter):
Would I get better gas mileage by buying 1/2 a tank of gas once a week versus traveling
around with a full tank for the first week after filling up? For the record there are numerous service stations in my neighborhood
pay cash when buying gasoline so the only inconvenience for me would be the time to takes to stop in and buy the gas.

Yes, but marginally. On average you'll be carrying around less weight (about 40 pounds for your car), and the gas in your car will be a little more fresh on average. But I'd imagine the MPG gain would be very low, all else equal. It might be hard to even notice over time. So it all depends on how you value your time. Personally, I wouldn't do it. It's like working 5+ minutes for a dime or two. I'll spend the spare change for another 5+ minutes of free time.

BreninTW · Thu Dec 03, 2015 12:53 am

With all the discussion about electrical systems and fuel consumption, it made me think of a question I've long had.

Are modern alternators clutched?

I've often heard it said that turning off electrical things increases fuel consumption, but I've never been able to figure out why that is! Granted, the only engines I'm particularly familiar with are pre-1985 petrol engines, but they all had alternators that were driven by a belt from the crankshaft. Didn't matter what electrical bits were on or off -- that alternator was always spinning, and thus generating current and resistance.

I imagine that modern hybrids use any excess current to charge the batteries ... but what about non-hybrids? For example, my car reduces fuel consumption by using electrical power steering (and possibly electrical air-con -- I've not got a firm answer to that one), so the alternator needs to be running pretty much constantly to power the steering and other electrical bits and pieces. When the stop-start kicks in, the steering immediately becomes extremely heavy, because there's no assistance as the electrical power is cut, and only the lower-consuming bits draw from the battery.

Aesma · Thu Dec 03, 2015 1:00 am

In my parents' Citröen C4 Grand Picasso II everything is electric and can run on the battery, so when the stop-start kicks in (under 8Kph when decelerating, as it's an automatic), you hear the engine stop but A/C runs, brakes and steering are not affected, etc.

The alternator is merged with the starter, too.

Without talking about disengaging the alternator, since you always need some electric power, I believe the load should cause it to offer more or less resistance depending on its level.

Okie · Thu Dec 03, 2015 2:48 am

Quoting BreninTW (Reply 24):
Are modern alternators clutched?

There is no need, the parasitic load for bearings is quite small and the energy to excite the rotor with no load even less.
That is the beauty of an alternator. The only time you put mechanical energy in to the alternator is when you take electrical energy out.
Now with computers running the vehicle, injector fuel pumps, cooling fans, ignition systems, a/c blowers etc there is just never a time when the engine is running that you are not requiring the alternator to provide electrical current.

I have always since the advent of in tank fuel pumps have just planned to fill at 1/4 tank. My personal opinion is that the pump does require cooling/lubrication of the fuel.
I have never lost a fuel pump on a road vehicle. I have witnessed friends and acquaintances that run them to empty and have noticed unscientifically that they have had fuel pump failures, my brother being one of that type.

I would be less concerned about the weight since it is more about how fast you plan to accelerate the mass (fuel mileage) or decelerate (brake wear).
Unless you are planning to drag race from street light to street light then I suspect the amount of fuel savings is going to not worth the effort.

Okie

NAV30 · Thu Dec 03, 2015 4:00 am

Quoting MSPNWA (Reply 23):
On average you'll be carrying around less weight (about 40 pounds for your car), and the gas in your car will be a little more fresh on average. But I'd imagine the MPG gain would be very low, all else equal.

We have warm-to-hot weather here, nine months of the year. I've always felt that less fuel in the tank equates to 'more air' - and that, since that air is likely often to be warm, a lot more fuel will probably evaporate (meaning that low fuel in the tank will tend not to last as long as fuel in a fuller one)?

[Edited 2015-12-02 20:41:55]

StarAC17 · Thu Dec 03, 2015 4:39 am

Quoting Osubuckeyes (Reply 2):
I'm guessing that the extra weight carried in car fuel tanks when filling to full is marginal compared to the extra weight an aircraft would carry filling to full.

Quoting diverted (Reply 3):
Any weight savings will lead to efficiency gains. How much a half tank of gas will save is probably pretty trivial in comparison to thousands of lbs on an aircraft.

There are a few things to note here.

An aircraft is in continuous operation from when it starts it's journey to it's end and unless you drive a lot your car wouldn't be in continuous operation. Furthermore the initial amount of fuel used to start a car will probably offset any gains from one additional start and stop cycle from multiple fuel trips as you have to turn the engine off.

Quoting dfwjim1 (Thread starter):
Would
this type of thing also work when it comes to cars and trucks? For example I drive about 150 miles a week (mostly freeway)

That isn't much and highway driving is the most efficient driving. It's worth a go to experiment on seeing if you would notice a difference. One thing to note is that most gas gauges do not drop at the same rate, it stays at full longer and then drops faster as it gets from about 3/4 to empty at the same rate. Half on the gauge might not be half in the tank.

To do an experiment you would have to have an empty tank and run it to empty and see the differences in mileage and everyone with any sense will tell you that running a tank to empty is a bad idea.

Quoting BreninTW (Reply 24):
I've often heard it said that turning off electrical things increases fuel consumption, but I've never been able to figure out why that is!

The electronics on a car when running are powered by the alternator which constantly charges the battery as the battery can only run them for so long you need more output from the alternator to run things. The stock stuff would probably not be noticed as much but if you put custom things in like a sound system you actually will have to upgrade the alternator and the circuitry on the car to make everything work.

Quoting tu204 (Reply 16):
But as you pointed out, this increase will be miniscule. My advice would be gradual accelerations. This would give you the most noticable fuel savings. To get an object to move from a standstill takes quite some power. Avoid unecessary braking followed by rapid accelerations.

Also check your TIRE PRESSURE!! Can't stress that one enough. Ensure that the tires when cold are at the specified pressure which can be found inside the driver's door, on the gas tank hatch or in the manual. Also if you live in an area that gets cold a block heater or parking the car in the warmest environment possible will ensure the car uses less fuel to start. The colder the engine is the more gas it uses.

einsteinboricua · Thu Dec 03, 2015 2:27 pm

Back when gas was over a dollar a liter (over $4/gal), I embraced hypermiling with my Caravan and later my other car. Made sure tire pressure was at its maximum allowed threshold, offloaded any weight I didn't need (in essence I wanted the back seats removed since I only carried my sister and myself but mother insisted I kept them. Her office junk was removed though), and switched to neutral whenever I went downhill and coasted to stops. I also took a page from the airlines and fueled up only when tank was below 1/4 or when there was a sudden drop in prices (in other words, made sure to use as much fuel as possible before loading up).

I was also very rigorous on not opening windows while cruising on the highway but after it was discovered that there's really no difference, out here in the states I've been more lax about it, especially when temperatures are between 50-low 70s.

One thing I did notice though was the quality of gasoline helped determined how much mileage I got. For instance, fueling up in the cheaper, independently owned chains, I found that a tank didn't last as long as refueling from a more well established brand (Texaco and Shell were my go-to chains).

Lots of things to consider, but this was just my experience.

Flighty · Thu Dec 03, 2015 4:00 pm

Quoting BreninTW (Reply 24):
Are modern alternators clutched?

Reading online it seems that current DOES vary to keep your battery properly charged. I think alternators are not clutched, as I've never heard of an alternator clutch before.

Accessories are moving from belt driven to electric. My 1990s Jeep has a belt driven fan (clutched), a belt driven power steering pump, a brake booster fed from that, and a belt driven A/C compressor that REALLY cuts down the power.

On newer cars, electrification has resulted in electric power steering pumps (kind of like the Boeing 787's electric flight controls) and for the same reason - fuel efficiency.

Electric fans are now pretty universal (huge power and efficiency savings). This started back in '89 or so. I remember we had an '89 saab with it.

And AFAIK electric A/C compressors are now used as well! I pulled up a picture of a 2010 Prius AC compressor. It appears to be electric (meaning it can run even with the engine off, too). There is no belt wheel.

So the only belt driven accessory you really need now is an alternator. This allows for fully functional hybrid vehicles to run with/without the gas motor.

na · Thu Dec 03, 2015 4:26 pm

Quoting Osubuckeyes (Reply 2):
I'm guessing that the extra weight carried in car fuel tanks when filling to full is marginal compared to the extra weight an aircraft would carry filling to full.

Absolutely, as fuel in a longhaul airplane can be 50% of its weight, or even more, while in an average car that weighs perhaps 1500 kg a full tank will add just about 70 kg.
A full tank in a car vs. half full is equivalent of carrying your twelve year old kid vs. driving alone. It may save you 1/4 of a liter over 100 kms on a middle class car, but hardly more.

vikkyvik · Thu Dec 03, 2015 8:22 pm

Quoting einsteinboricua (Reply 29):
and switched to neutral whenever I went downhill and coasted to stops.

I'd think that would actually use more gas. For example, in my stick shift cars, shifting to neutral while slowing down or going downhill will force the engine to keep pumping gas, in order to keep it at idle.

Leaving it in gear allows the engine to reduce or cut off gas flow, and let the car's movement keep the engine turning (also allows engine braking).

Dreadnought · Thu Dec 03, 2015 8:33 pm

Quoting dfwjim1 (Thread starter):
Would I get better gas mileage by buying 1/2 a tank of gas once a week versus traveling

Of course you would have better milage, but is it worth the effort? Let's say your tank holds 16.5 gallons. That's 100 pounds of fuel. you'd be carrying 50 pounds less for half the time, so average 25 pounds less cargo. According to the gub'ment which says 1% per 100 pounds, you might get 0.25% better fuel economy (more if you have a small car, less if you have a bigger one - but then your gas tank size would also vary.

https://www.fueleconomy.gov/feg/driveHabits.jsp

All in all, not worth the effort. But I would recommend dumping a lot of the trash that most of us seem to accumulate in our trunks.

WildcatYXU · Thu Dec 03, 2015 8:55 pm

Quoting Flighty (Reply 30):
Electric fans are now pretty universal (huge power and efficiency savings). This started back in '89 or so. I remember we had an '89 saab with it.

That's because most of current cars have transversely installed engines and driving the fan by belt would be a bit tricky. I don't think there would be too much power saving compared to a clutched fan.

ikramerica · Fri Dec 04, 2015 4:29 am

You should always make it a practice to keep your tank at least half full. In the case of a natural or other disaster you will regret having an empty tank. This is especially true in tornado and earthquake prone regions as these can strike without warning.

NAV30 · Fri Dec 04, 2015 11:21 am

Quoting einsteinboricua (Reply 29):
switched to neutral whenever I went downhill and coasted to stops.

I'd strongly recommend that you leave the car in gear in those circumstances, einsteinboricua - in case you need to make an emergency stop. 'Engine drag' is an advantage in an emergency (like a kid running out into the road), and helps you stop shorter........especially downhill.........

[Edited 2015-12-04 03:25:34]

einsteinboricua · Fri Dec 04, 2015 12:42 pm

Quoting vikkyvik (Reply 32):
I'd think that would actually use more gas.

Quoting NAV30 (Reply 36):
I'd strongly recommend that you leave the car in gear in those circumstances, einsteinboricua

You guys didn't read when I said

Quoting einsteinboricua (Reply 29):
Back when gas was over a dollar a liter (over $4/gal)

In other words, I don't do it anymore (then again, Missouri is relatively flat).

NAV30 · Fri Dec 04, 2015 12:55 pm

Quoting einsteinboricua (Reply 37):
In other words, I don't do it anymore

Fair enough, einsteinboricua.

But are you saying that, if (when?) gas prices go up again, you'll revert to 'free-wheeling'? I sure hope not?

Flighty · Fri Dec 04, 2015 1:38 pm

In manual transmission cars, free wheeling is a part of normal driving. Yes, especially in 2008 era when I was poor, and gas was 4.30 I was getting into that. Also cutting way down on trips.

CXfirst · Fri Dec 04, 2015 2:14 pm

Quoting stealthz (Reply 21):
Correct, 2x55w Low beam halogen bulbs = 110w. Approximately 1/7 HP

That energy has to come from somewhere, ultimately the fuel tank.

There is a top gear episode somewhere where they are roadtesting one of the Audis after they got the LED daytime running lights (at least I think it was the Audi). As part of the show, there read a quote from the brochure, saying that the LED lights were fuel saving compared to the normal daytime running lights.

Top Gear posed the questions to Volvo, and Volvo apparently calculated it. And, IIRC, the resulting fuel savings of the LED vs ordinary daytime running lights, was absolutely minuscule.

The above, is purely just recollection, can't remember exaclty if Volvo compared daytime running lights vs LED, or if it was vs no lights at all.

Anyway, might not be the most reliable info, but anyway, if you are saving fuel by not using lights, you are better off finding savings elsewhere. I personally think all cars should have daytime running lights. Definitely makes them easier to see!

-CXfirst

NAV30 · Fri Dec 04, 2015 3:10 pm

Quoting Flighty (Reply 39):
In manual transmission cars, free wheeling is a part of normal driving.

Fascinating, Flighty. In Commonwealth countries, it's the direct opposite - kids are taught to re-engage the 'drive' as soon as they've changed down or up........

Odd feeling really - I spent most of my career helping to build new towns - two of which were close to a high-speed road called the A1, which connected London and Edinburgh. Then I got sick - and when I (more or less) recovered, I decided to give up 'town development' and instead helped a friend to run his driving-school business.

It suited me in many ways - I'd always been a teacher 'first and foremost,' and I think I can claim to have been quite good at teaching driving. Never had a pupil fail the driving-test, anyway.

Point is, though, that the LAST thing we'd have allowed the kids to do was 'free-wheel' - they either changed down or up, and then let the clutch back in. No way would we have encouraged them to 'free-wheel,' asking for trouble in my opinion?

Are kids in the USA taught to drive with the clutch 'out' for any distance? If so, that's just plain dangerous in my view; as I said, just 'asking for trouble'? The car would simply be 'out of control' and they'd be pre-occupied with holding the clutch-pedal down?

[Edited 2015-12-04 07:14:55]

WildcatYXU · Fri Dec 04, 2015 3:17 pm

Quoting NAV30 (Reply 41):
Are kids in the USA taught to drive with the clutch 'out' for any distance?

Clutch? What? Where?

I seriously doubt kids in the USA are taught to handle the clutch - the driving instructors would have to be able to drive a manual car first.

Flighty · Fri Dec 04, 2015 3:21 pm

Quoting NAV30 (Reply 41):
Are kids in the USA taught to drive with the clutch 'out' for any distance? If so, that's just plain dangerous in my view; as I said, just 'asking for trouble'? The car would simply be 'out of control' and they'd be pre-occupied with holding the clutch-pedal down?

Kids in the USA do not learn to drive manuals. We are around 98% automatic as a market.

I think it is just a driving "skill" to remain in control while coasting. Sometimes you go a mile down a very long hill. Sometimes engine drag is desired; sometimes it is a hindrance. There are many times when you might slow, then accelerate.

Hyper milers would coast, go a bit too fast in the middle, then avoid the second acceleration, saving fuel. This can be done if you know your car and the roadway perfectly. But no, we are taught to accelerate, brake, accelerate, brake. And never shift out of D or out of gear.

I was told coasting would wear out the "throwout bearings" (?) but 80k miles so far and no problems.

NAV30 · Fri Dec 04, 2015 3:32 pm

Quoting Flighty (Reply 43):
I think it is just a driving "skill" to remain in control while coasting.

Meaning driving an 'automatic' without the drive being engaged, Flighty?

The next best thing to 'attempted suicide,' IMO, Flighty......

Flighty · Fri Dec 04, 2015 5:21 pm

I don't see a problem.. If I am going into a blind corner in second gear, I can't stop without first declutching. The car will keep pushing forward, maybe hitting a child around the bend (in that sort of emergency, the engine kills, and somewhat impedes stopping). The streets are narrow where I live. If no acceleration is needed, and there is a possibility of braking then the right thing to do is declutch I find (?). But I would not shift an automatic into neutral, no.

Revelation · Fri Dec 04, 2015 5:24 pm

Quoting Flighty (Reply 43):
Kids in the USA do not learn to drive manuals. We are around 98% automatic as a market.

I learned to drive in the late 70s at a driving school and even then they did not offer training on manuals. They didn't want to deal with worn out clutches. The students wanted to get the license as fast as possible and there was no requirement to be able to drive a manual so they were happy to get going with the automatic.

I think the 'tipping point' must have been the late 60s. All the cars my parents owned from the 60s and earlier were manual, 70s and later were automatic.

That being said, it was always thought to be 'cool' to drive a manual, so kids sought out a way to learn it. In my case, I was given too much time on my hands when I had access to an employer's vehicle so I used that opportunity to learn how to drive manual. Hope they forgive me! These days I find my BMW's automatic transmission is better at shifting than I am so I'm happy to let it do it most of the time, and when I want to bang through the gears myself it has a way to let me do that too!

That does point out a good way to save fuel while on the highway is to use cruise control. Us humans are too imprecise with our feet. The computer keeps a nice smooth speed whilst we tend to waste fuel accelerating and decelerating by small amounts despite our best effort to be smooth.

Another tip is to go through your car (especially glove box and trunk/boot), take EVERYTHING out, and only put back the things you can truly justify bringing with you EVERYWHERE you go. Each pound/kilo does add up over time, and chances are if you haven't done this in a while you'll find many pounds/kilos to get rid of.

I got these tips from a radio interview with people whose hobby it is to compete on achieving the best gas mileage. Yes, there really are people who do that for a hobby, just like there's people who go stand in the cold near the airport and take pictures.

Aesma · Fri Dec 04, 2015 5:24 pm

Quoting StarAC17 (Reply 28):
Furthermore the initial amount of fuel used to start a car will probably offset any gains from one additional start and stop cycle from multiple fuel trips as you have to turn the engine off.

If that were true then stop&start systems wouldn't exist. With my parents' car, in the city, the engine shuts down all the time. Since it happens while the car is still going forward, sometimes I might need to accelerate after having decelerated (at a roundabout for example, or a red light that turns green while I was getting to it), then the engine will be started by the starter-generator, while engaging the 1 or 2nd gear at the same time. So basically even though the car isn't hybrid, I'm electrically powered for a moment.

Quoting Flighty (Reply 30):
On newer cars, electrification has resulted in electric power steering pumps (kind of like the Boeing 787's electric flight controls) and for the same reason - fuel efficiency.

There is no pump, it's an electric motor on the shaft or driving the rack. The fuel savings are in part thanks to not needing to continuously drive a pump, but instead providing electric power only when needed.

Quoting Flighty (Reply 39):
In manual transmission cars, free wheeling is a part of normal driving. Yes, especially in 2008 era when I was poor, and gas was 4.30 I was getting into that. Also cutting way down on trips.

Free wheeling with the engine shut down maybe, otherwise I'm not convinced. Energy efficient driving is taught in France since a few years (my sister just got her license) and free wheeling isn't part of it. On the contrary it's ground to fail the test.

zckls04 · Fri Dec 04, 2015 5:26 pm

Quoting NAV30 (Reply 41):
Fascinating, Flighty. In Commonwealth countries, it's the direct opposite - kids are taught to re-engage the 'drive' as soon as they've changed down or up........

Freewheeling in neutral is fine if you're not on a hill though (e.g. when approaching slow traffic which is imminently going to start moving).

Revelation · Fri Dec 04, 2015 6:23 pm

Missed a few things earlier...

Quoting tu204 (Reply 16):
This power comes from where? You generator.

Around here we use the term alternator rather than generator. Generator implies the generation of direct current rather than alternating current that gets fed to a regulator to become direct current.

Quoting MSPNWA (Reply 23):
Yes, but marginally. On average you'll be carrying around less weight (about 40 pounds for your car), and the gas in your car will be a little more fresh on average. But I'd imagine the MPG gain would be very low, all else equal. It might be hard to even notice over time. So it all depends on how you value your time. Personally, I wouldn't do it. It's like working 5 minutes for a dime or two.

And for most of us, the energy consumed by detouring to the gas pump and restarting the engine would overwhelm the savings. In my case the gas station is only one extra traffic light and two blocks drive out of my way, but in the past it would have much more than that.

Quoting Aesma (Reply 25):
In my parents' Citröen C4 Grand Picasso II everything is electric and can run on the battery, so when the stop-start kicks in (under 8Kph when decelerating, as it's an automatic), you hear the engine stop but A/C runs, brakes and steering are not affected, etc.

The alternator is merged with the starter, too.

Now THAT is like a 787. The same windings that get driven by electricity to start the engine in turn get driven by the engine to generate electricity.

I wonder what kinds of tradeoffs that incurs. You really need a lot of torque at low RPMs to start a cold engine, yet once the engine is started the RPMs will be high. Is different gearing used for the two different modes?

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