Here's how you get tire diameters:
245/45/18
245 / 25.4 = 9.6457
9.6457 x
0.45 = 4.340
4.340 x 2 = 8.680
8.680 +
18 = 26.68
26.68 is the overall tire diameter
For MPH tire speed based on RPM:
26.69 / 26.00 = 1.0261
1500rpm / 1.0261 = 1461
1461 / 13.211 = 110mph
These are very basic easy to use laymen "equations". Your actual overall tire diameter will vary based on vehicle weight, tire psi and quite a few other variables. For a more accurate RPM to MPH numbers you will need engine speed, transmission gear ratio and differential gear ratio.
I hope this helps..
