If you’re looking to change your rims and/or tires, you can use the calculator below to see how the size of your new tires compares to your current set and can give you an idea of whether they might rub and the impact on the speedometer reading.
This calculator uses the nominal tire size based on the specifications provided by the user and doesn’t take into account tire wear or air pressure effects. The results provided are an estimate, any decisions made based on these results are made at the risk of the user.
Calculation and Assumptions
Example 205 x 55 R16
The first three digits are the tire width in millimeters, i.e. 205 mm. The next two digits represent the profile height (sidewall) as a percentage of the width, i.e. 55% of 205 mm gives a profile height of 112.75 mm. Couple of interesting oddities now, the R16 is the diameter (not radius as the R might suggest) of the Rim in inches (just to mix up the units a bit?), i.e. rim diameter 406.4 mm.
Thus the overall diameter of the tire is the rim diameter plus two profile heights, i.e. 406.4 + 2 x 112.75 = 631.9 mm (24.878 in).