Tire Size Calculator

mm
First number in tire code (e.g. 225 in 225/55R17).
%
Middle number in tire code (e.g. 55 in 225/55R17).
in
Last number in tire code (e.g. 17 in 225/55R17).
Everyday Life

Calculate tire dimensions from a standard tire size code (e.g. 225/55R17) including outer diameter, circumference, and revolutions per mile.

Formula

Sidewall = width × aspect%; Outer diameter = rim + 2×sidewall (in); Circumference = π × diameter
  • Tire code: WIDTH/ASPECT R RIM (e.g. 225/55R17 = 225 mm wide, 55% aspect, 17-inch rim).
  • Sidewall height = tire width × aspect ratio ÷ 100.
  • Revolutions per mile = 63,360 ÷ circumference (in inches).

225/55R17

Inputs
  • Tire Width: 225 mm
  • Aspect Ratio: 55 %
  • Rim Diameter: 17 in

Sidewall = 225×0.55 = 123.75 mm = 4.87 in. Outer dia = 17+2×4.87 = 26.74 in. Circumference = 84.0 in. ~753 rev/mile.

Frequently asked questions

What does 225/55R17 mean?
225 = tire width in mm; 55 = aspect ratio (sidewall % of width); R = radial construction; 17 = rim diameter in inches.
How does tire size affect speedometer?
Larger tires rotate less per mile, making speedometer read low. Smaller tires read high.
What is a plus-size upgrade?
Plus-sizing increases rim diameter while reducing aspect ratio to maintain the same overall diameter (to avoid speedometer error).
How much does tire size affect fuel economy?
Wider tires increase rolling resistance slightly. Taller gearing from larger tires can improve highway fuel economy.