Ah - we're probably just playing with words here, but really, the effective diameter or radius is what is reduced by squish. It really does not matter what the rest of the tire looks like, the only thing that's important is the distance from the spindle to the ground. That governs how many revolutions the PCM will see in a mile (or the effective circumference).
I think the "squish" effect SHOULD be applied to both sides, because as far as circumference goes, it's 2 x pi x radius, and the radius is reduced the same way on all sides of the tire (as it touches the ground).
If you calculate the "squished" part of the radius, it would be: (unloaded tire radius - rim radius) x squish, which is then added back to the rim radius to get the squished radius. This is multiplied by 2 to get the diameter and then by pi to get the circumference.
You get the same result with this calculation: (unloaded tire diameter - rim diameter) x squish + rim diameter, which gives the diameter (adjusted for squish) directly.
If you DON'T separate the non-deforming rim from the rubber, then a 3% reduction gives about the same result.
- Jack
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2014 F150 Platinum SCrew 3.5L EcoBoost 4x4 with SCT programmer
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