Dang, I hate these non-standardized ways of measuring things. Black, the conversion ratio is EXACTLY 25.4 mm/inch which gives an unloaded circumference of 2792.875869 mm or closer to 2793 mm (if we want to quibble over one millimeter).
Personally, I've found that a 3% reduction in that total circumference is about right, since the wheel itself does not "squish", only the rubber part does, and Bill's calculator keeps the wheel radius constant. So, 2793 - 3% = 2793 - 84 = 2709 mm.
In my case, I found a 3% squish in total radius was about equal to a 7% squish in the rubber part. So, if I use Bill's calculator, which wants "rubber squish", I enter a squish value of 7%. This of course changes depending on your tire construction, wheel diameter, and inflation pressure. My tires are inflated to 40 psi, and I have 275/65-18s.
So, Slowpoke, I'd start with 2709, and, Black's advice is dead on - this is just a starting point. You have to try a setting and compare it to a GPS or radar gun measurement to get it right.
Hope I'm not confusing things here.
- Jack
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2024 F150 Platinum SCrew 3.5L PowerBoost FX4, Peragon Tonneau Cover, LineX Bed, 35% Window Tint on All Sides and Rear, Full Nose Paint Protection Film, Husky Mud Guards, Lasfit Floor Liners, VIOFO Dash Cam