I have recently discovered that there is a 5mph discrepancy between the speed that the speedo indicates and the speed that my Satnav indicates.

This isn't just in the car I'm currently driving, but also in my g/f's car too, where there is a 4mph difference. (In my own car however, the 2 speeds are identical).

I have a feeling that possibly the car is made to indicate a higher speed on purpose, to save you in cases where you are caught speeding. 100mph in the UK for example is an automatic ban, so if I have 100mph on the speedo, I'm only actually doing 95mph so would just get points and a fine.

Which is the more accurate, the car or the Satnav ?.

 

Quoting Leezyjet (Thread starter): Which is the more accurate, the car or the Satnav ?.

I'd be more inclined to side with the GPS, provided you are traveling at a steady rate.

I'd try testing your car's speedometer with another model GPS. If you get the same result as you did with your GPS, then the issue is with your speedo.

Quoting Leezyjet (Thread starter): This isn't just in the car I'm currently driving, but also in my g/f's car too, where there is a 4mph difference. (In my own car however, the 2 speeds are identical

Can I assume the car was showing as slower?

The reason I ask is that as tires where their diameter gets smaller and therefore they don't cover as much distance as they did when new per turn.

I've noticed the same difference -- even before I had a GPS receiver in my car, testing my speedometer against things like automated speed enforcement/reminders (i.e. the "Your Speed XXX Speed Limit YYY" trailers that local police departments sometimes drop off to slow people down revealed a ~5 MPH difference (the delta is slightly lager the faster I'm going)

I'm inclined to trust the GPS over the speedometer since the speedometer accuracy can be affected by tire size, inflation, temperature, etc.

Not to mention none of the other gauges on my car are particularly accurate -- I can go at least an 1/8" past the Empty line on my gas gauge [There was an article in the Wall Street Journal about this a few months ago -- aparently you shouldn't try this in Germany where car owners expect and car manufacturers ensure that "Empty" means "Empty" ]

Lincoln

SatNav

In the UK it is legal for car speedo's to underread by 10%, and this is probably what is happening - I know at motorway speeds mine shows about 5mph below the actual speed of 70.

In Sweden and probably in most countries it is illegal to have a speedometer that shows a too slow speed. So the manufacturers play it safe.

Quoting PlaneWasted (Reply 5): In Sweden and probably in most countries it is illegal to have a speedometer that shows a too slow speed. So the manufacturers play it safe.

Sorry, my post should have said overread!

You're speedo is probably offset to make you think you're going faster then what you actually are. The GPS is probably correct, but it'd be incline not to rely on it unless you have a receiver which is super sensitive.

Car speedos do read differently. I was once doing 60mph and overtaken by a car doing 58 (both digital speedos)

I don't get the question?

Quoting Myt332 (Reply 9): I don't get the question?

Ha ha !!. Is that you in 20 years ?.  

Quoting ShyFlyer (Reply 1): I'd try testing your car's speedometer with another model GPS

I just got a GPS based speed camera detector that shows speed, and this compares with the Sat Nav.

 

The car speedometer is much more likely to be accurate. They are required by law to be within 5% of your true speed. The electronic sensors used to determine speed from axle rotation are brutally simple and accurate to several significant digits (if desired).

The GPS used in commercial navigation systems are not that sophisticated and have surprisingly low accuracy. In many situations, a navigation unit can only find your location within about 10-25 meters and then makes an educated guess as to what street you are driving on based on compass heading and approx speed. That is a considerable margin of error.

Satellite navigation in general is a bad source for velocity data. There's a reason military and high-performance systems still use inertial guidance and accelerometers as primary guidance and GPS as supplemental guidance.

[Edited 2008-08-11 12:15:22]

I found that on my old car, the speedo was about 5 MPH slower than either what the radar signs or the GPS would say. On my current car, the difference is less than 1MPH and is probably not statistically significant.

Quoting DfwRevolution (Reply 11): The car speedometer is much more likely to be accurate.

I respectfully disagree. With 4 cars of my own, and all the rental cars I've driven, seldom do two agree. I have found the newer the car, the better, but overall there is a tendency to read higher than actual speed. A while back in California, a gentleman kindly verified the accuracy of my GPS with his radar, and it only cost $300! What a deal, eh? Both read 74 mph.
Car speedos, especially of the old fashioned mechanical type, are prone to errors, and when you throw in replacement tires of a different rolling diameter, the speedo is not terribly accurate. Also I have noted that some have a more or less fixed error, whereas some others have a larger error the faster you go.

Quoting Leezyjet (Reply 10): Ha ha !!. Is that you in 20 years ?.

20 years! Your being generous.

According to the German non-profit consumer organisation Stiftung Warentest, Navigational systems were rather inaccurate and should not be used for checking speed limits.

http://www.mz-web.de/servlet/Content...&atype=ksArtikel&aid=1213961249983

I somewhat doubt that, but Speedos are indeed allowed to show a little more than the real speed. They must also show the correct speed for different tyre sizes approved for the car. While a speedometer correction is required by law for tyres which have a significantly different diameter, some deviations between the approved tyres do occur. Therefore, a speedo cannot be 100% accurate anyway, as almost every car has different tyre dimensions approved.

In any case, a speedo is an extremely simple construction. My 1983 VW bug uses the same system a VW bug from the 1940s used, and it isn't less precise than many modern instruments. While modern speedos use a digital system to measure the speed, the principle hasn't changed.

Quoting TheSonntag (Reply 15): While modern speedos use a digital system to measure the speed, the principle hasn't changed.

I'd argue that point, too. (I'm in that kinda mood today). Early speedos use a cable to spin a magnet, which in turn tries to rotate a cup which has the pointer attached to it. A spring restrains the cup, so the faster the spinning, the farther the magnet can turn the pointer. A crude, yet effective mechanism. Modern speedos use a pickup that counts pulses from a geartooth, and then either drive a digital display, or an analog pointer. Two very different ways of calculating speed.

Quoting Avt007 (Reply 13): I respectfully disagree. With 4 cars of my own, and all the rental cars I've driven, seldom do two agree.

First of all, I said it's only a likelihood that a speedometer will be more accurate. I purposefully left the door open for rare cases where a speedometer is malfunctioning or is poorly calibrated.

Quoting Avt007 (Reply 13): With 4 cars of my own, and all the rental cars I've driven, seldom do two agree. I have found the newer the car, the better, but overall there is a tendency to read higher than actual speed. A while back in California, a gentleman kindly verified the accuracy of my GPS with his radar, and it only cost $300! What a deal, eh? Both read 74 mph.

That all sounds suspiciously subjective...

Quoting Avt007 (Reply 13): Car speedos, especially of the old fashioned mechanical type, are prone to errors, and when you throw in replacement tires of a different rolling diameter, the speedo is not terribly accurate

Speedometers must be accurate within a certain margin by law whereas there is no such requirement for GPS navigation systems. Look into how both devices are measuring velocity and there is no question that the properly-functioning speedometer is the more accurate device.

As I said before, civilian GPS systems are not accurate to more than 10 meters. So imagine a car starting at x = 0 meters at time 0 seconds traveling at 30 m/s (about 67 mph). With up to 10 meters of error, the GPS could read an initial position corresponding to a true coordinate of x = -10 m. Two seconds later the car is at coordinate x = 60. But with error, it's possible to read x = 70 m. So GPS would read 80 meters displacement over 2 seconds. That's 40 m/s, an error of 33% !! The legal error of 5% for a speedometer would have shown 31.5 m/s.

If one were teaching an engineering class, this would be a great example of where an analog system would outperform a digital system.

The GPS is what will be right. I know what in my car the speedo is 3mph+. I know this for a few reasons. I have heard that all VW are 3mph+, the radar screens that show how fast you are going on the side of the road show 3mph+. My speed warning which is set to 80mph goes off at 83mph and my limited top speed of 130mph is an indicated as 133mph on the speedo. My car even knows that the speedo is 3mph+. Why its like that I dont know.

Quoting DfwRevolution (Reply 17): Speedometers must be accurate within a certain margin by law whereas there is no such requirement for GPS navigation systems. Look into how both devices are measuring velocity and there is no question that the properly-functioning speedometer is the more accurate device.

I won't argue that -when manufactured- or perhaps even -when originally sold- automobile speedometers are required to have a certain accuracy, however, I can tell you that in nearly 10 years of owning my car I've never been required to have the speedometer recalibrated -- or even checked -- either as part of the registration renewal, emissions testing, or any of the other things that I am required by law to do to my car on a regular basis.

(I will say that every police crusier I've sat in the driver seat of [but never driven] I've noticed the notation on the speedometer legend "Calibrated" and my understanding is that -- at least in California, those speedometers are recalibrated on at least an annual basis. I suspect that may have to do with the use of "pacing" as a speed enforcement technique and the need to ensure that the speed you claim someone is going is accurage, but I digress)

I will also argue that if you're measuring distance using a spinning round object, the distance traveled is affected by the diameter of that spinning object -- which can be affected from (duh) the diamater of the tire, to tread wear (though I doubt that by itself would be a major difference) to inflation and even temperature (as air in tire gets warmer, it expands, increasing the diamater of the tire), etc.

On the other hand my GPS (A Garmin StreetPilot something-or-other) has no moving parts, and [generally] has at least three points to reference its position off of and is remarkibly accurate positionally, as long as it has a clear view of the sky [Downtown Cleveland, for example, can confus it] -- I've marked points of interest and it has returned me to the _exact_ spot (+/- about 10') on a consistent basis -- Cleveland has it's share of whacked out streats (intersections going 5 or 7 different ways, streets closely paralleling other streets, etc.) and with few exceptions it's managed to figure out the street I'm really on vs the one on the other side of the hedge going the same direction.

Every time I've tested my StreetPilot and Speedometer against a speed measuring device, the StreetPilot has been dead on, and my car has been around 5 miles over -- for example, sign on side of road tells me I'm going 64, Garmin tells me I'm going 64, Speedometer tells me I'm going 69. Every time I've done the test I've had the same result.

Quoting DfwRevolution (Reply 11): The car speedometer is much more likely to be accurate.

Uhm no, its far from being accurate.

All cars have purposely inaccurate speedometers, all the reasons why have been mentioned already, and I could mention plenty more reasons why not.

Recalling from my advance avionics class...

A GPS is extremely accurate for speed. I don't have my manual with me but my Garmin handheld was accurate down to around .001mph IIRC, and this is just a wimpy handheld.

While GPS receivers need at least 3 (sometimes 4) satellites to get a position, for speed you only need 2. Even in the worst days, 99% chances are you're going to get at the very least 5 satellites in view at a given time. and 90% of the time you're gonna have more than 6. The more satellites you have, the more accurate it is (ok yeah, there are exceptions, but that's another can of worms) The only drawback, and this varies with what receiver your using, is the refresh rate. Most handhelds refresh every second, which lets be honest, is plenty often enough. Some airborne units, IIRC, are less than a second (around 40hz). The GPS signal, in layman's terms, is really just a timed signal. That's why every satellite has 3 atomic clocks on board, accurate to 1 second every 100000 years.

Look online for speed tests and what not, you'll see most car review magazines, manufacturers, etc rely on GPS units purpose built for speed measrument.

Quoting DfwRevolution (Reply 17): civilian GPS systems are not accurate to more than 10 meters.

Of course they are, and with WAAS, it's down to 1m/3ft.

[Edited 2008-08-11 19:04:47]

Quoting Lincoln (Reply 19): my GPS (A Garmin StreetPilot something-or-other)

I've got the Garmin StreetPilot too. I also have the optional traffic alert system (well when it can pick up the FM signal - as it didn't today and left me stuck in a traffic jam covering 4.7 miles in 57 mins !!!) attatched.

Seems yours is as accurate as mine too then, which it seems is more accurate than the car speedo !!.

 

Quoting DfwRevolution (Reply 11): The GPS used in commercial navigation systems are not that sophisticated and have surprisingly low accuracy. In many situations, a navigation unit can only find your location within about 10-25 meters and then makes an educated guess as to what street you are driving on based on compass heading and approx speed. That is a considerable margin of error.

While you are correct about the absolute accuracy of civ. GPS receivers, this has no relevance to calculating velocity. 10-25 M "accuracy" isn't a measure of noise or jitter in the results at any given time, errors are very consistent and change very gradually. If they didn't, systems like WAAS would be useless.

Quoting DfwRevolution (Reply 11): Satellite navigation in general is a bad source for velocity data. There's a reason military and high-performance systems still use inertial guidance and accelerometers as primary guidance and GPS as supplemental guidance.

Not true strictly speaking. I worked on a military project that used GPS for primary guidance inputs and the guidance package did not include intertial guidance or accelerometers. We did supplement the GPS with tilt sensors and a digital compass but these were only used for determining the relative wind and package attitude.

We could operate this with civilian or PLGR (military) GPS units and there was no difference in velocity accuracy but a very noticeable difference in absolute location accuracy. In either case, the biggest errors we normally saw were due to operator error when coordinates were entered using the wrong GPS datum.

Quoting DfwRevolution (Reply 17):
Speedometers must be accurate within a certain margin by law whereas there is no such requirement for GPS navigation systems.
Because a speedos is not allowed to show too little but allowed to show too much, the manufacturers make them show too much. The error allowed is pretty big also, between 5 and 10 % if I remember correctly.

Quoting FlyMIA (Reply 18):
Look into how both devices are measuring velocity and there is no question that the properly-functioning speedometer is the more accurate device.
Over inflated tires, under inflated tires, weight of the car, outside air-pressure, speed, surface the car is travelling on and temperature will affect the speedometer readings.
That's why they put some margin and make the speedometers show too much.

[Edited 2008-08-12 23:25:38]

I dont know about speeds, my wee motor car is built for pootling. But I do know that people trying to find my address using sat-nav get lost. The post code directs them to a completely different road and in a warren like I live that is very confusing.

I wouldn't trust sat-nav for anything. Learn to read a map and plan a route - or is that something people cannot do these days?

And if you get nicked speeding, it isn't the sat-nav that will be used in evidence against you, it's your car's speedo. Especially doing over the ton! Of course, they have to catch you first..................  

Andy   

Quoting Lincoln (Reply 19): I've noticed the notation on the speedometer legend "Calibrated" and my understanding is that -- at least in California

All police-packaged vehicles (Chevy Impala, Tahoe, Dodge Charger, Ford PI), regardless of its assigned jurisdiction, have a Certified Calibration label on their speedometers when they roll out of the assembly line. My guess is that these vehicles undergo a more stricter test by the manufacturer vs. non-police models before being delivered.

Quoting Lincoln (Reply 19): my understanding is that -- at least in California, those speedometers are recalibrated on at least an annual basis.

My guess is that any recalibration testing is subject to the policies of the law enforcement agency. In short, not every jurisdiction recalibrates their patrol vehicle speedometers.

Quoting Skidmarks (Reply 24): I wouldn't trust sat-nav for anything.

I use mine lots- it's the greatest thing since sliced bread.

Quoting Avt007 (Reply 26): I use mine lots- it's the greatest thing since sliced bread.

Ditto that   !

Unless I know where I'm going (i.e. have been there before) I tend to use my GPS. It's fantastic because I can be basically anywhere in the continental US (possibly Canada, I'm not sure though) and get directions from where I am to where I want to be -- even when I don't know where I want to be (when I'm on the road, for example, I may be hungry -- and I may know that I want steak or a burger or whatever -- but I haven't the foggiest clue what my options are in the area... my GPS can tell me.)

Of course you view the directions with a bit of skepticism and not expect true door-to-door (99% of the time I'm within a 1/4 mile, about 75% of the time I'm within a few hundred feet), but mine has only lead me astray a couple times, usually due to the place I'm trying to get to being newer than the database on the GPS (and in those cases I can give the GPS a nearby intersection and figure out the rest from there)

Lincoln

It is so damn useful if you spend a lot of time like me, in a rental car, in a city I've never been too, trying to find a customer, a place to eat, whatever. Plus I go exploring the city more than before, because I know I can find my way back to the hotel, even in the pitch black and torrential downpours, like my last trip to PDX.

Definately the sat nav! I can bring up a digital speedo in my car and it is 4mph slower than the dial. Sat nav gives you a true (albeit slightly delayed) reading of your speed.

Quoting Daleaholic (Reply 29): Sat nav gives you a true (albeit slightly delayed) reading of your speed.

How the hell can it be true if it is delayed? You want to know what speed you are doing BEFORE you hit the camera!! lol not after!!  

Andy   

Quoting Skidmarks (Reply 30):

Not all of us are old with bad legs   We can keep our speed constant!

My Garmin Nuvi 250 is accurate to my Honda Civic with + or - 1 km/h, so either its accurate or both the GPS and the car say you are going faster than you actually are.

Thankfully now knowing the speedo is over by 5mph saved me from being pulled over today. I was doing 34mph on the speedo, in a 30mph zone and drove past some cops hiding in bushes with a radar gun !!. Thankfully on their gun I would only be doing 29mph !!!.

That has totally proved (well to me anyway) the Sat Nav is the more accurate of the 2 (even though I didn't actually have it with me on this occasion), otherwise I'd be nursing 3 points and a £60 fine right now !!.

Quoting Skidmarks (Reply 24): I wouldn't trust sat-nav for anything. Learn to read a map and plan a route - or is that something people cannot do these days?

I can do both very well, the sat nav is quicker and easier and offers more but I always have a road atlas in the car as back up. The Sat Nav is much better for cities too, where it would be impractical to have A-Z's of every city littering the car. My Sat Nav also covers most of Western Europe too.

Just the other day, I was returning from Scotland following the Sat Nav route as I hadn't been to that part before. It seemed to be taking us on alot of A roads and I wondered when we were going to join the motorway (we were coming down from West Kilbride back to Yorkshire). I looked at the route the sat nav was taking, and it was taking us down the coast road more or less to join the M6 near Gretna. I double checked the Atlas and found if we stayed on the road we were on for 20 miles more, it would take us to the M74/M6 which would be alot quicker than being on an A road, even though the A road was a more direct route !!.

Alot of people are too stupid to question the Sat Nav and will just blindly follow it, thats why we hear of cars driving into lakes and being stuck in allyways. Not alot the Sat Nav can do about the stupidity gene !!. But you don't always have to go the way it takes you, it will automatically re-calculate to get you to the destination if you go another way - even better with the traffic add on to route you around jams too !!.

 

Cars almost always read "Optimistic"

This makes it look like you're going faster. So you say "OMG 65mph is so quiet in this new Pontiac" while in reality, you are going 60 MPH.

Also, it makes the car seem to get more miles per gallon (Here, 8% faster speed = 8% longer distance for the same fuel burned).

Also, it protects customers from getting tickets.

So those are 3 strong reasons why car manufacturers purposely make the speed-o-meter read about 6-8% fast or high. There are exceptions but this is very common.

Quoting Lincoln (Reply 19): for example, sign on side of road tells me I'm going 64, Garmin tells me I'm going 64, Speedometer tells me I'm going 69. Every time I've done the test I've had the same result.

Me too. I have one near my house where 33mph reads 38MPH on the dash. It's like, thank you digital sign, I didn't realize I could afford to go a good deal faster, making things more dangerous! Sweet!

Quoting Flighty (Reply 34): There are exceptions but this is very common.

My Tribute must be one of those amongst the exceptions then. Its speedometer has been dead on accurate since the day I bought it nearly 7 years ago.

Now the speedometer on my Kymco scooter is exactly 5mph optimistic. It took me a while, but I now automatically compensate for the discrepancy.

Quoting Flighty (Reply 34):

The easiest way to test the accuracy of one's speedometer would be to find a stretch of highway that has mile markers on it and maintain 60 MPH (which equals 1 mile/minute) between two of those markers while timing it. If one clears the next mile marker in exactly one minute; they're definitely going 60.

Quoting Leezyjet (Reply 33):

Don't get me wrong Leazy, I can see the usefulness of a sat nav in finding your way to somewhere new, or to a precise point. However, they are not always accurate (as is evinced by my visitors turning up a mile away). To use the speed indicated by the sat nav rather than the car speedo would seem to me to be rather daft unless it has been properly calibrated to the car.

Most aircraft are strictly controlled over the instrumentatation, cars not so. And a bolt on sat nav that isn't part of the avionic suite would not be the most reliable I would have thought.

Anyway, I'll stick to winding up the nutters who believe their sat nav gives them the right to swerve across the road from the fast lane to turn off by getting in their way, reading my map  At least when I read my map, I am aware of the need to turn at some point and prepare for it.

Oh, and I don't drive lorries down narrow roads and get stuck under bridges cos the sat nav tells me to.   

Andy