Now you are making my brain hurt! The entire unloaded circumference of the tread *is* hitting the ground with each revolution. The only thing I can think of is that the tire distorts and scrubs at the contact point to make up the difference between that and 2 x pi x r.
Perhaps, this makes sense...As you say, for our purpose, the diameter of the wheel/tire starts with the perpendicular measurement from the center hub to the ground. Here, we're determining the effective
radius. The flat portion of the tire, created under load, effectively reduces the circumference of the tire by a length equal to the radian length we lose from the length of tread in contact with the ground. In other words, by cutting a line through the edge of the circle, we remove that resulting length from the total circumference to get the effective circumference. So, the number of revolutions per foot or per mile, etc. is
actually increased as a result of creating this constant flat spot in the tire; and, the flatter the tire, the shorter the effective radius, the shorter the effective circumference...and, the more revolutions we get over the same ground distance. I'll spare everyone by stopping there. I'm surprised I haven't already drawn fire for this as it is, but this seems like a great group on the forum.
I'm kinda confused about this gear hunting that is mentioned. I've read that the computer is "smart" and adjusts to your driving habits. But what does it adjust exactly and why? If it is hunting for gears, that might be because has yet to adjust to your new tire size. I'm planning to build an offroad camper on mine, so it will need to make adjustments that are a lot bigger than what it needs for a tire change! Like a lot more weight and aero resistance.
I'm assuming there's code that acts upon the input of engine RPM's, load and speedometer readings, all of which is affected by gearing as well as all the stuff we've been discussing. I'd think an "unexpected" wheel/tire size could influence the computer's "decision" as to when to upshift/downshift. Not sure I see the rationale behind "hunting" per se, but I'd expect for automatic shifting to potentially occur at different-than-normal RPM's, if more weight is placed on wheel speed? I don't know. Again, I haven't noticed any big issues and am not complaining. Mainly just curious.