Speed Display Accuracy ?

[lydgate]

Does anyone know how accurate the speed display in Navigator is? The speedometer in my car (Peugeot 307) reads about 3 - 4 mph faster than the speed readout in TomTom Navigator

[Dave]

Pretty accurage. The speedo's in-car are usually calibrated, but not very well, so you'll usually see them up to 4mph out.



Put it another way, I've had 4 GPS Receivers showing the same speed in-car, and then I've had my Speedo. You will see errors when accelerating because GPS is usually 2 seconds behind, but when you're at a constant speed it's pretty accurage.

[TonyP]

Now that SA is off, the accuracy is around 0.1mph with a good fix, maybe dropping to 0.5mph with a degraded signal.



That's only on the level though, consumer GPS units only report 2D velocity, so on a hill you are travelling slightly faster. On a 1 in 4 hill, 15%, about the steepest you get, you will doing 3.5% more than what the GPS says, eg, 31mph if the GPS indicates 30mph.



On a more normal 5% hill, the difference is only 0.5%, or 0.25mph at 50mph

[agibbs]

Remember also that the car speedo is counting wheel revolutions so it has to know the distance covered each time. As the tread wears your wheel diameter gets a cm or two smaller so this will slowly increase the reported speed by quite a few %. As there is a tendency for high speeds to be over-reported by the speedo anyway it is not unusual for a car on balding tyres saying nearly 80 mph to be barely doing 70. (Although in the wet this is probably a good thing!)



Trust the GPS!
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Andrew Gibbs

[hollis_f]

All car speedos are calbrated to read 5-10% higher than actual speed. Car speedos are not very accurate devices. If they were accurately calibrated then it would be possible for you to be nicked for breaking the speed limit while your speedo showed you were save. By deliberately mis-calibrating in this fashion the manufaturer removes the possibility of being sued by people caught out by the errors in their speedos.



It's the baker's dozen of car manufacturers.

[TonyP]

On my car the speedo is quite accurate upto about 100kph (60mph) After that it starts to over read so that an indicated 150kph (in Germany) is only 142kph.



The speedo on my wife's car is almost spot on, even at high speeds.

[Mark G]

Has anyone else noticed that the odometer seems to measure mileage with better accuracy than the speedometer measures speed. My TTN calculated distances seem to be very close to my odo' measurements, a far better match than the speed measurements.

[Clive]

The car odometer used to be directly geared to the turning of the speedo cable and so assuming you have the right size tyres fitted will be quite accurate. The speedo needle is dragged around by a rotating magnet and so to be on the safe side is usually set to read fast.

This does not always apply to the modern electronic speedos.
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Clive

[julia]

I have noticed the same thing, Odometer correct speedo wrong. All UK cars I have tried seem to over read but when I go to the US the speedo is usually spot on with the GPS. Even the cruse control goes up in 1.0 mph steps.

[aero]

I drove from Cornwall to South Wales, TTN2 indicated the journey would be 181.1 miles. I set the trip computer in my car to count down from 180 miles and when I arrived the trip reading was -0.1 miles!!!!!!!! Remarkable!



I find the car speedo reads (more or less) a constant 3mph too fast (at any speed) cf GPS.

[TonyP]

I have discovered that the EU regs for speedos say that they must read in the range -0% to +10% + 4kph. ie they must never underread. The modern electronic instruments are probably accurate to around 1% so obviously EU constructors are obliged to set the speedos at about 5% too fast in order to be sure they are in the range. There is no law on odometer readouts so they can be spot on.



The US has no law on speedo readings, so the constructor can set the speedo as close as possible, ie +/- 1%

[DavidW]

Tony's post accords with my understanding - that car speedometers are not allowed to show the vehicle at travelling than less than the true speed, but can show the vehicle at travelling at up to 10% faster than the true speed.





The situation is different for vehicles that, by law, have to carry a tachograph. A tachograph installation has to be calibrated and sealed by a registered installation centre. Driving a vehicle with a defective or unsealed tachograph head or sensor is an offence under most circumstances, as is having defective or unsealed speed limiter equipment on vehicles that are required by law to carry it. There are, of course, some limited exceptions to this to allow for example, for vehicles where the installation becomes faulty during a journey.



I don't know what the precise requirements are for tachographs, but they are required to be a lot more accurate than a car speedometer! I believe I'm right in saying that the calibration has to be done on a rolling road.



I wouldn't be surprised if the tachograph specification is similar in that the vehicle can't be shown at travelling at less than its true speed - though I suspect the amount it's allowed to show at more than its true speed is much less. This is a guess, however - and unlike the past, I'm not currently in touch with any PCV / LGV fitters or licence holders.





My Zafira's speedo is about 8% fast at many speeds. A GPS 50mph is about 54mph on the speedo. This is an electronic installation - the odometer is digital.



It'd be interesting to see how accurate the trip computer's average speed indication is - I can easily run at a steady speed over a few hundred yards to see as I have cruise control. The trip computer uses the same speed sensor as the speedo so far as I know.







David

[aero]

My car's computer has a speed warning utility so sounds an alarm if you exceed a pre-determined speed. The analogue speedo is reading half way between 80 and 85 mph when set to go off at 80mph.



But at lower speeds it still reads 2-3mph out, so it seems that the speedo is accurate but just set to read slightly too fast?



I believe the car's computer measures speed differently to the speedo - something to do with ABS sensors. I'm a bit vague about this though.

[TonyP]

On modern cars (last 10 years) all the speed measurements are derived from an electronic pulse on the wheel or gearbox. The accuracy depends on the computer knowing the circumference of the tyre. That can vary a bit with tyre pressure and speed, and it decreases slightly as the tyre wears, by about 1%.



On my car, all the computer functions other than instant seed are more or less spot on. The odometer, average speed, fuel consumption are all accurate. The instantaneous speed shown on the computer, however is 5% fast and is exactly the same as the speed shown by the speedo needle. The speed limiter is also 5% fast.



The manufacturer has actually calibrated the computer to read 5% too fast to conform to EU regs. I am sure that the same model in the US will have a speedo that reads spot on.

[lbendlin]

Don't worry folks, here in the US the speedo have the same accuracy "issue". They too show about 5% too much at most times (not that it would be of any importance to anyone here ;-)



Dave,



no wonder your four GPS receivers show the same speed - they all get their same wisdom from the satellites. I don't think any GPS program will go through the hassle of calculating speed on their own. Most all of them will take it from the NMEA data VTG



Lutz
_________________
Lutz

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