Can't speak for the others, but for me it is driven by a craven search for
truth. Underinflation, 185SR14's, and calibration errors can easily conspire
to the tune of an optimistic 10 to 15%. Translates to the same in odometer
accuracy. Fuel economy maps, etc.

Anecdotal exchange with another list member: Changed tires, mileage dropped
by 12%. There follows a discussion of friction, tread slip, possible belt
misalignment. I suggested he check the tire pressure. It was at 35 psi all
around. Vanagon savvy tire specialist. He was running 195/70x14 passenger car
skins. 15% easy. Remember, revs per mile for a given tire are a function of
pressure, casing temperature and load. Effective rolling diameter.

Ignorance and superstition are the enemy. The search for measurable empirical
truth is good. Like the perceived dangers in outrageous rpm's. Diogenes,
Socrates, hemlock.

Frank Grunthaner