http://www.miata.net/garage/tirecalc.html
If you have a look at a few tyre web sites they will actually give you the physical dimensions of the tyre vs. whats on the sidewall, two different tyres will be physically different to each other in size about 98% of the time.
Also there's approx 6mm of wear (or 12mm across the diameter) on each and every tyre so the speedo is going to be inaccurate at some time or from the very first day the tyres are fitted.
Most cars speedos read fast by a few % regardless.
p.s. good on ya Bogus . . .
