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Originally Posted by JDLarue
I just talked to a Hypertech tech on the phone about that today. He said he just measures the tire from the top to the ground or take the radius and multiply by 2. Either way is accurate.
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accurate wouldn't be accurate
Do you use the radius from the hub to the bottom, top, sides?
Measuring the height of the tire while on the vehicle should provide an incorrect number, albeit a who-cares kinda incorrect.
When determining the speed of the vehicle and mileage, it obviously measures how much the circumference of the tire travels. Wouldn't matter if the tire was oblong running on tracks, the circumference is what matters, not the diameter or height.
Since it actually is NOT completely oblong but the bottom is flattened due to vehicle load, it's still the same association. Lift the tire, take the circumference and divide by Pi and that should provide the most accurate number to use for programming when changing tire sizes. Especially since tire companies list their tire sizes as they sit off the vehicle, not under load.