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More Accurate - Sat Nav Or Speedo?  
User currently offlineLeezyjet From United Kingdom, joined Oct 2001, 4042 posts, RR: 53
Posted (6 years 1 month 1 week 5 days 14 hours ago) and read 4045 times:

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 ?.

 Smile


"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"
38 replies: All unread, showing first 25:
 
User currently offlineShyFlyer From , joined Dec 1969, posts, RR:
Reply 1, posted (6 years 1 month 1 week 5 days 14 hours ago) and read 4035 times:



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.


User currently offlineL-188 From United States of America, joined Jul 1999, 29800 posts, RR: 58
Reply 2, posted (6 years 1 month 1 week 5 days 14 hours ago) and read 4032 times:



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.



OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
User currently offlineLincoln From United States of America, joined Nov 2004, 3887 posts, RR: 8
Reply 3, posted (6 years 1 month 1 week 5 days 14 hours ago) and read 4032 times:

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



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User currently offlineMhodgson From United Kingdom, joined Dec 2002, 5047 posts, RR: 25
Reply 4, posted (6 years 1 month 1 week 5 days 14 hours ago) and read 4027 times:

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.



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User currently offlinePlaneWasted From Sweden, joined Jan 2008, 525 posts, RR: 0
Reply 5, posted (6 years 1 month 1 week 5 days 6 hours ago) and read 3952 times:

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.

User currently offlineMhodgson From United Kingdom, joined Dec 2002, 5047 posts, RR: 25
Reply 6, posted (6 years 1 month 1 week 5 days 1 hour ago) and read 3915 times:



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!



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User currently offlineBill142 From Australia, joined Aug 2004, 8451 posts, RR: 8
Reply 7, posted (6 years 1 month 1 week 5 days 1 hour ago) and read 3909 times:

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.

User currently offlineOly720man From United Kingdom, joined May 2004, 6737 posts, RR: 11
Reply 8, posted (6 years 1 month 1 week 5 days ago) and read 3902 times:

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


wheat and dairy can screw up your brain
User currently offlineMyt332 From United Kingdom, joined Sep 2003, 9112 posts, RR: 70
Reply 9, posted (6 years 1 month 1 week 4 days 23 hours ago) and read 3890 times:

I don't get the question?




One Life, Live it.
User currently offlineLeezyjet From United Kingdom, joined Oct 2001, 4042 posts, RR: 53
Reply 10, posted (6 years 1 month 1 week 4 days 16 hours ago) and read 3834 times:



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

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

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.

 Smile



"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"
User currently offlineDfwRevolution From United States of America, joined Jan 2010, 977 posts, RR: 51
Reply 11, posted (6 years 1 month 1 week 4 days 15 hours ago) and read 3820 times:

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]

User currently offlineDocLightning From United States of America, joined Nov 2005, 19724 posts, RR: 58
Reply 12, posted (6 years 1 month 1 week 4 days 13 hours ago) and read 3781 times:

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.

User currently offlineAvt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 13, posted (6 years 1 month 1 week 4 days 13 hours ago) and read 3774 times:



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.


User currently offlineLuv2fly From United States of America, joined May 2003, 12110 posts, RR: 48
Reply 14, posted (6 years 1 month 1 week 4 days 12 hours ago) and read 3767 times:
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Quoting Leezyjet (Reply 10):
Ha ha !!. Is that you in 20 years ?.

20 years! Your being generous.



You can cut the irony with a knife
User currently onlineTheSonntag From Germany, joined Jun 2005, 3595 posts, RR: 29
Reply 15, posted (6 years 1 month 1 week 4 days 12 hours ago) and read 3756 times:

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.


User currently offlineAvt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 16, posted (6 years 1 month 1 week 4 days 11 hours ago) and read 3745 times:



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.


User currently offlineDfwRevolution From United States of America, joined Jan 2010, 977 posts, RR: 51
Reply 17, posted (6 years 1 month 1 week 4 days 11 hours ago) and read 3744 times:



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.


User currently offlineFlyMIA From United States of America, joined Jun 2001, 7175 posts, RR: 9
Reply 18, posted (6 years 1 month 1 week 4 days 10 hours ago) and read 3735 times:

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.


"It was just four of us on the flight deck, trying to do our job" (Captain Al Haynes)
User currently offlineLincoln From United States of America, joined Nov 2004, 3887 posts, RR: 8
Reply 19, posted (6 years 1 month 1 week 4 days 9 hours ago) and read 3719 times:



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.



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User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 20, posted (6 years 1 month 1 week 4 days 8 hours ago) and read 3710 times:

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]

User currently offlineLeezyjet From United Kingdom, joined Oct 2001, 4042 posts, RR: 53
Reply 21, posted (6 years 1 month 1 week 3 days 13 hours ago) and read 3633 times:



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 !!.

 Smile



"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"
User currently offlineBHMBAGLOCK From United States of America, joined Jul 2005, 2698 posts, RR: 5
Reply 22, posted (6 years 1 month 1 week 3 days 8 hours ago) and read 3597 times:



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.



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User currently offlinePlaneWasted From Sweden, joined Jan 2008, 525 posts, RR: 0
Reply 23, posted (6 years 1 month 1 week 3 days 4 hours ago) and read 3583 times:

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]

User currently offlineSkidmarks From UK - England, joined Dec 2004, 7121 posts, RR: 55
Reply 24, posted (6 years 1 month 1 week 3 days 4 hours ago) and read 3577 times:

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.................. wink 

Andy  old 



Growing old is compulsory, growing up is optional
25 PHLBOS : All police-packaged vehicles (Chevy Impala, Tahoe, Dodge Charger, Ford PI), regardless of its assigned jurisdiction, have a Certified Calibration lab
26 Avt007 : I use mine lots- it's the greatest thing since sliced bread.
27 Lincoln : 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
28 Avt007 : 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, wh
29 Daleaholic : 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 delay
30 Skidmarks : 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
31 Daleaholic : Not all of us are old with bad legs We can keep our speed constant!
32 StarAC17 : 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
33 Leezyjet : 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 pa
34 Flighty : 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 real
35 Flighty : 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
36 ShyFlyer : 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.
37 PHLBOS : 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 (whic
38 Skidmarks : 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 al
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