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...when you compare calculated tire diameter (Dc) versus actual tire diameter (Da), using various tire manufacturer's published Revolutions-per-Mile (rpm), you'll find that 0.97 -- or 3% deformation -- seems to be a typical / common value seemingly regardless of actual tire size, make or manufacturer.
Here's the basic formula to use which assumes NO deformation:
MPH = [ 60 / (G × A) ]×[ RPM / rpm ]
...where:
MPG = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-mile.
60 = conversion constant, minutes-per-hour
G = transmission gear ratio (usually highest gear)
A = axle ratio
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