Calculating Tire Diameters

(Quoted from Michael Kaczmar and the DSM archives)

Here is the way to figure tire diameters:

Section width x one tenth of the aspect ratio. Think of the aspect ratio as a percentage. This will give you the wall thickness in milimeters. Change that number to inches, by dividing by 25.4 and then muntiply by 2 and add the rim diameter. This should give you the theoretical tire diameter.

For example:

195/50-15 - 195 x 50% (or 0.5) ... 195 x 0.5 = 97.5mm. Multiply x 2 (you have two wall thicknesses in the overall diamter = 195mm. Convert to inches: 195/25.4 = 7.677".

Add this to your wheel diameter: 15 + 7.677 = 22.677 ... For government accuracy you are looking at a 22 - 5/8" tire diameter.