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Accuracy of speedometer and odometer? Why not just check them with a GPS?
If you are comparing fuel economy, best to make sure both cars are going the same speed, otherwise, what have you proved? With my car, a one mph error in speed makes a one mpg change. I apply corrections to both speed and odometer when it's important to know.
If you increase tire diameter, fuel economy will usually increase, but it will SEEM to go down unless you correct for guage changes.
/Ernie Rogers
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