http://www.discounttire.com/dtcs/infoTireMath.dos 

Technically speaking… if one is going to do some calculations…

The odometer/speedometer is effected by the rolling radius of the tire… I.e. the distance between the road and the axle. A 30 inch tire with a 1,000 pound load will have a static radius of less than 15 inches between the axle and the road. The distance a tire will travel per revolution with, say, 1,000 pound loading will be less than the measurement around the UNLOADED tire.

Raising the tire pressure will reduce the speed indicated by the speedometer.

Lowering the air pressure in a rear (front for FWD vehicles) tires will cause the odometer to record more miles.

The generic factory and tire manufactures pressure recommendations do not take into account the many variables in actual vehicle usage. And then most of us change tire sizes and it all goes out the window.

The best tire pressure to run is very complex subject.

It would be nice if our members would add their vehicle information to their signature (similar to mine) plus add their City and State . A member in the same town might be willing to drop by and help you with a problem. Or drink all your beer!