Surely modern day tyres are more consistent than that, no?
If you are measuring wheel speed with a laser tach, I don't think tyre pressure would effect that.
It's not a matter of consistency. It's about the improved traction with radial ply tires which have more flex in the sidewalls to allow the tread to remain flat on the road when the car is subjected to lateral force as in a turn or cross wind. That same flex allows the tire tread to wander more than the more rigid bias ply sidewalls do.
The next time that you drive on the highway, note how many times that you have to make very tiny steering corrections in a mile of straight road. With bias ply, it's maybe 4 or 5, with radial ply, more like 4 or 5 every thousand feet.
Wheel speed is dependent on the rolling radius which is set with tire pressure and the load in the tire. Again, the flex of the sidewalls will also affect the rolling radius when small imperfections in the road surface, or lateral forces, cause the tires on one side to flex more than the tires on the opposite side.
The differences are tiny, but the cumulative effect is noticeable with the small incremental steering inputs required of the driver to maintain position in the lane.
Please, don't forget that there is a difference between highway speeds and LT/GT speeds. At the low speeds of bias ply tractor tires running on the dead smooth surface of a tank, a laser tach would be more precise, but not really necessary. The same tire pressure should be adequate for tires of the same size.
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