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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.


For a speedometer, the roll-off will give the practical numbers needed for accurate guage measurement, but I think these numbers will still differ from a theoretical calculation using factory specs. In the contact patch of a tire, the amount of tread that normally fills an arc is compressed into a line from the front to the rear of the tire's footprint. Wouldn't that compression lead to a smaller distance travelled (my tires look like they lose almost an inch in diameter) when rolled as opposed to, say an unmounted tire of the same size?

1992 Mighty Max 4x4
3.0 5-speed
Herculined bed