Tire dimensions assume proper inflation and no load, so yes - the vehicle's weight slightly distorts the dimensional characteristics of the tire, depending on air pressure.... easy to see with a flat tire. <img src="/forums/images/graemlins/wink.gif" alt="" /> The distortion is (by design) primarily in the cross sectional dimension (sidewall) rather than the circumference.

At low speeds, the tread circumference represents the total distance covered in one tire revolution. At high speeds, centrifugal force and developed heat causes the tire to "grow" slightly... the faster it spins, the more it grows, causing the distance covered by each tire revolution to slightly increase.

The GM manual calls for a change in ABS chip with tire size changes of more than 10% from stock. Their recommended method for measuring tire diameter is to mark the tire and pavement on a level spot, move the truck forward through several complete revolutions and divide the total distance travelled over the ground by the number of revolutions.

The slight distortion caused by vehicle weight on a properly inflated tire won't change the distance covered with one revolution. Think of a v-belt in either a circle or around a set of pulleys - the distance travelled by the outside edge of the belt is the same with one revolution of either example.

Frank