5RWill suggested I post my question in here, so copied it and posted it below.
Some folks have been talking about tires, and my question is; is there a way to tell the computer that the tire diameter has changed? I have suspected that folks experience mileage loss on a bigger tire may be due to not accounting for it in the speedo and therefore the trip calculator shows a drop. The reason I say this is that when a 31" tire travels 5280 feet (a mile), it gets there in 650 revolutions, but a 33" tire gets there in 611 revolutions (not accounting for slippage or bumps on the road).
So a truck with the smaller fires would still be traveling when the one with the big tires completed the mile (all things being equal).
Is my logic sound?
Of course, you can let the GPS tell you what speed you are traveling at and compare that to the speed-o-meter. If you are traveling faster than what the speedo shows, then you are also covering more ground. And that has to be accounted on the mileage calculation (and that goes even if you ignore the co puter and do the Ol trusty "fill-up-tank-then-travel-a-distance-and-then-fill-the-tank method).
GerryLP