every calculator I have looked at has a flaw!!!!
They assume the overall diameter is the same as the rolling diameter AND THIS IS NOT TRUE!!! The rolling diameter is about 3% smaller than the overall (unloaded) diameter.
No, it's not the same as the loaded diameter (or better specified as the "Static Loaded Radius". This is because the tire is not only "squished" at the bottom, but it "hangs" from the top. ("Squished" "Hangs" Surely there are some sweet sounding technical terms that cover them, right? I wonder what they are?)
So it's better to think of "rolling circumference". This way it's easy to understand that the belt seems to act like a tank track with the tread rubber on top of the "links" - and it is the distance between each link times the number of links, that determines the "circumference". What this means is that so long as the calculator is doing comparisons, the "3% problem" isn't a problem.
But if they start calculating Revs per mile or gear ratios, there's a built in 3% error - except for this tire calculator!!! Kudos, 1010Tires!!
(Full Disclosure: These guys talked to me when they were revising their web site.) (OK, nit pick time!! They still didn't get the width right. It should read "Section Width", not "Tread Width")