| Diameter | Radius | Circumference |
| 30" | 15" | 30p = 94.24777961" |
| 31" | 15 1/2 " | 31p = 97.38937226" |
The circumference represents how many inches the tire will travel for ONE revolution.
Therefore the circumference (in inches) must be converted to miles.
94.24777961" * 1 '/12" * 1 mile / 5280 ' = .0014874965 of a mile
97.38937226" * 1 '/12" * 1 mile / 5280 ' = .001537097 of a mile
Now we consider the rate 55 mph calibrated for the 30" tire. D = R*T
55mph/ .0014874965 of a mile = 36974.87638
36974.87638*.001537097 = 56.83397156
So if you enhance your tire to a 31" diameter your speedometer should read approximately 57 mph.
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