Compare Two Tire Sizes Side by Side

The first number is the section width in millimetres, the second is the aspect ratio as a percentage of that width, and the last is the rim diameter in inches. R means radial construction.

A common guideline is to stay within 3% of the original overall diameter. Beyond that the speedometer error grows and you may run into clearance, load-rating or gearing problems.

A speedometer counts wheel rotations and is calibrated for the original tire. A larger tire turns fewer times at the same speed, so the speedometer reads low; a smaller tire makes it read high.

Plus sizing fits a larger rim with a lower-profile tire so the overall diameter stays close to original. The calculator lets you check that the diameters match within the 3% limit.