I've done the GPS check a hand full of times. A few times with the GPS on my wife's cell phone and a few times with a handheld Garmin. They both told me the truck was travelling 10 mph faster than what the speedometer was reading i.e. a speedo reading of 50 had the GPS telling me I was running 60 mph. Of course this is sort of subjective with a speedo needle that is wavering ever so slightly.
Now, the Garmin also has a distance readout. So twice I compared my work commute distance (43 miles) with the GPS and the trucks odometer. To my surprise the odometer reading over the 43 miles agreed to with the GPS within a tenth of a mile each time.
I think of the speedometer error in terms of an offset, since the odometer seems to be pretty accurate. And I don't think I'd need to adjust my MPG numbers because the trucks odometer is accurate.
Denny, it would be interesting to know how your odometer readings compare to GPS readings.
