[wimorrison]

Originally Posted by jimbg

I had a long debate on another thread about mpg, the fuel consumed figure will be very accurate but the distance measurement is the area of possible concern.

But, does the speedo over-reading actually correlate to any error in the distance traveled? Or is the speedo over-reading independent of the distance run?

Need to take a GPS on a very straight road on cruise control to compare speed.

Not sure how to check distance accuracy?

The speedo is only a reflection of the number of times that the wheel has rotated and thus the distance travelled for each rotation of the wheel. The car is 'programmed' to expect a set distance for each revolution of the wheel and as the tyre wears you travel less distance though the car thinks you have gone the same distance and because of this the fuel consumption will appear to get worse as the car thinks you have used more fuel to cover the theoretical distance. The wear effect will be amplified with under-inflation, reduced with over-inflation ~ perhaps this over-inflation effect is the reason the high pressure (39PSI) is called the 'eco mode'

As you suggest a GPS is needed to remove the tyre variation in the distance traveled and speed - many GPS will give you both instantaneous speed and distance travelled from a set point.

@volvorocks - clearly I missed your posts on the subject and I seek forgiveness.