accuracy of our speedo

[leasaunce]

i went for a long cruise yesterday on he Freeway, 400km both ways. set the cruise control to 100kph, but according to TOMTOM my actual speed is 92kph.(I even asked my mate, who was driving in front of me set his cruise on 100kph as well, his car the golf gti, STOCK)

Did 120kph on the speedo, but on Tomtom only 109kph.
So i tried low speed, 60kph on speedo, 57kph on tomtom.

is this normal on the gti? I've got this car brand new 2 weeks ago. everything is stock.

[VW GTI]

Yep.

http://www.cartoday.com/content/magazine/roadtests/pdfs/2006/12/01/poloVSfiesta.pdf

[Small_Fryz]

that would explain why every man and his dog was passing me when i was doing 110kph on the speedo in a 110kph zone.... and i sat on 100kph on the highway and every man and his dog was passing me then too....

Looks like i need to get a GPS of some sort to get correct indications

[firdausasri]

Any car speedometer will definitely overread, I'd say 8-10%. Using normal GPS (without differential) will not be accurate either because of signal and multipath issues. I've used a industrial spec GPS with Omnistar real time differential to test this issue, you'll see difference from 80km/h onwards

[VW GTI]

that would explain why every man and his dog was passing me when i was doing 110kph on the speedo in a 110kph zone.... and i sat on 100kph on the highway and every man and his dog was passing me then too....

Looks like i need to get a GPS of some sort to get correct indications

GPS costs more than getting your speedo recalibrated. But recalibrating the speedo doesn't tell you where to go.

Any car speedometer will definitely overread, I'd say 8-10%. Using normal GPS (without differential) will not be accurate either because of signal and multipath issues. I've used a industrial spec GPS with Omnistar real time differential to test this issue, you'll see difference from 80km/h onwards

I've had mine tested on a dyno. Wrong by around 5% all the way to 160. I've got 17s on mine so it's been corrected a bit.

[PeterHBne]

This is what I do. Get your trip computer to either the 1 or 2 setting (depending on whichever you don’t care gets reset often). Go to the average speed option. Pick a speed - say 100KMH indicated, hold on cruise control then reset the average speed setting using the reset button on the stalk and it will give you an accurate KMH reading. I find invariably, as mentioned elsewhere, indicated 100kmh is about 92-94kmhs actual and 110 indicated is about 102 actual. After a while you will get to learn what your actual speed is against indicated.

[Bubu]

I used a GPS (non industrial) to calibrate the speed on the Scanguage. Now I dont even bother looking at the speedo I just go off the readings of the Scanguage.

[nomodlowkmgti]

What rpm on the tacho is 100 gps speed maybe you can use a rev speed in 5th if it points to a mark as a way to do 100 real speed or is each polo a bit different.may be a formula to do with diff and 5th gear ratio x rpm to = 100.