lokokkee

Just checked the speedo reading of my MY97 Turbo against a Garmin Etrex GPS and found the former to be over-reading by about 10% at around 100 kph (62 mph), i.e GPS shows only 90 kph when speedo is reading 100. My tyre size is 205/45/16, so it has got a rolling diameter that is about 2-3% smaller than standard, but there's still quite a big difference. Which is more reliable?
Cheers.

submannz

Speedo's are difficult to work out exactly. You can get two cars from the same factory and manufactured straight after each other both have different speedo readings. The manufacturers only have to adjust the speedo's to be within 10% error.

As your tyres wear slightly the speedo will show to be slightly faster.

The only true way to get a speedo check, get a speeding ticket or put the car on a dyno.

Dan

lokokkee

A consolation is that the error is over-reading, so unlikely to get a speeding ticket if I keep to the indicated speed limit. Well, there goes my 24 m.p.g. cumulative fuel consumption figure. Got to take it with a pinch of salt now.

sooby

GPS equipment can have positioning errors. The equipment I use (aviation) have position error indicators. They also read in knots - Approx 1.15mile = 1knot. Many of the GPSs I've seen can switch between knots and statute miles - wouldn't have it on the wrong setting would you?

lokokkee

Sooby, the GPS is set to k.p.h., same as the car, so no setting error there. There is a positional error depending on the number of satelites received and their signal strength, but since this tends to be random and the readings were taken over a fairly long stretch, the error should cancel out.

I have just done a RR dyno (only 200 hp at the engine), but the operator didn't point out any error in the reading, maybe he wasn't looking for it.

Shouldn't have done the tests, now the performance figures of the car don't look that hot any more.

sooby

Lokokkee,
No the error needn't cancel out - it can be exaggerated. It depends on the satellite grouping - how the acquired satellites are spread across the horizon. In non-IFR rated aircraft this can and does lead to several hundred feet vertical displacement - that's scary when flying if you're silly enough to only depend on that!

I believe that the car's accuracy can only legally be a very small percent out but don't know the exact figure - but I was surpassed at how small!

I tried this in my car (MY00 turbo) with a Garmin GPS and it proved quite accurate. However, I would trust your cars accuracy over your GPS.