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I live in the UK. I recently bought a used car, which was an import from Japan. I am not the first UK owner. At the time of import, some conversion/adaptation took place so that the information displayed on the dash (speed, odometer reading, trip meter) is shown in miles (Japan uses km).

I recently had a chance to compare these readings with an independent GPS-based application running on the vehicle head unit. What I noticed is this:

  • Speedometer shows the speed slightly above that of GPS at low speeds, but slightly below GPS at high speed (e.g. vehicle's 70 mph corresponds to about 72-73 mph in GPS app, yet vehicle's 30 mph correspond to about 28 mph in GPS app)
  • Trip meter shows readings slightly higher than GPS. E.g. on one recent trip, GPS app showed total distance travelled of 19.8 miles, while vehicle trip meter showed 20.4
  • Vehicle odometer shows about the same difference between in distance travelled as trip meter

I connected a bluetooth OBDII unit to the vehicle's port and paired it with the app running on the vehicle head unit. One of the parameters the app shows is the speed difference between ECU and GPS, updating it in real time. As I accelerate from 0 to 70 mph, the value of the difference goes gradually from 3 to -3.

I have done such comparisons several times at different locations.

So, my questions are: is this behaviour normal? And if not, then should I worry about it?

  • I do note bends and loosing the radio connection cause the gps to show an incorrect speed often - once it has a good signal it re-establishes itself... – Solar Mike Jun 6 at 14:25
  • GPS is only accurate on a straight flat road, hills and curves will show a reading slower than actual. – Moab Jun 6 at 15:40
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It's perfectly normal for a speedometer to show a different reading to a GPS. It's also perfectly normal for the accuracy of a speedometer to vary more than the accuracy of a GPS varies. GPS aren't 100% accurate either but it would be fair to say they're potentially more accurate than a speedo, but probably less so than a calibrated speedo (fitted to certain police cars).

Whether your speedo works by spinning cable or by electrical pulses from the ABS the basic premise is the same, that a needle turns (in increasing speed direction) against a spring (forcing it in the decreasing speed direction) and the effort required to overcome the spring force may well vary depending on a few factors. Your particular speedo looks like the spring is less effective at low speeds and more effective at high speeds

Because a speedo is essentially a gauge of the number of wheel rotations (a prescribed distance) that occur in a certain time, anything that upsets the number of wheel rotations that occur as your car travels along the road will affect the speedo reading. Even tire wear will reduce the rolling circumference of the tire and have a minor percentage effect, but by far the biggest cause of overall inaccuracy in speedo readings is fitting tires that have a different circumference than the speedo was designed for. This does apply a uniform error though (eg a tire with a 10% larger circumference will cause a 10% lower speed to be declared) and there are websites that will calculate the difference in readings to be expected if different sized tires are fitted than specified by the manufacturer

It's typically not permitted from a legal viewpoint, to have a speedometer that declares you are going slower than you truly are (and it would also be fairly antisocial to run someone over in a car doing 40mph, doing them significantly more injury because your gauge of speed was claiming 30mph)

So your questions:

  • Yes, it's normal
  • It rather depends on how you feel about the fact that your speedo under-reads; this is illegal in your country, where a speedo may read anything between 0% under true speed and 10% over true speed before the annual MOT test declares it defective
  • Thanks! I do expect the speedo to be off a bit. What about the odometer recordings? – Aleks G Jun 6 at 13:58
  • The odo is either mechanically geared, or electrically linked to the same system that drives the speedo; they both count wheel revolutions (distance), it's just that the speedo presents it as distance travelled in a certain time, whereas the odo presents it simply as distance travelled. If your speedo is inaccurate your odo will be too, though the odo will be unaffected by variations introduced by the spring on the needle. Your odo over-reads, hence claiming your vehicle goes faster (drives further in a certain time) than the GPS does. the error is about 3% over on the odo; within limits – Caius Jard Jun 6 at 14:04
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The value reported by the vehicle is dependent on factors that don't affect the GPS. The gps is independent of all vehicular attributes like tire size, tire inflation, gear selection, etc. The GPS is simply measuring time and distance through space and calculating velocity. The value provided by the car is dependent on the sensors and systems of the vehicle to provide a calculated value.

Which one is right? Hard to say. GPS has accuracy limitations that are legislated by government, but I think those limitations are less than vehicle speed calculation limitations. Vehicle speed accuracy is also legislated by governments, there is an allowable amount of inaccuracy that is acceptable.

GPS was originally developed as a military tool and created to a much different standard than automotive speedometers.

That said, My current vehicle is the first one where the GPS and the vehicle match exactly. All other vehicles I've driven with my gps were not in sync. When I changed the tires/wheels on the current vehicle the gps and car are about 1/4 a MPH different (digital speedo in the car, so it's easy to compare). I've had previous cars with different wheels/tires, differential gearing, different transmission. The speedo was pretty useless in that car.

As a general rule comparing two values that are derived in different ways to different standards are unlikely to match exactly. It's the story of life!

  • Thanks! I do expect the speedo to be off a bit. What about the odometer recordings? – Aleks G Jun 6 at 13:58
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Perfectly normal, I'm yet to drive an analogue Speedo car that agrees with GPS.

Have your wheels been changed from standard?

An inch either way will alter your Speedo by 2mph unless the drive convertor was changed at the same time.

As for MOTs the car sits on the ramp for the entire check so the tester will never know if it's accurate or not.

  • Thanks. As for the MOT, it did successfully pass one last September :) – Aleks G Jun 6 at 15:07

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