Tire Size Conversion Calculator

A tire sidewall code like 205/55R16 contains three key numbers:

205, section width in millimeters (the widest point of the tire) 55, aspect ratio as a percentage (sidewall height is 55% of the section width) 16, rim diameter in inches

From these you can calculate everything else.

Sidewall height

sidewall_height (mm) = section_width × aspect_ratio / 100 For 205/55R16: 205 × 0.55 = 112.75 mm = 4.44 inches

Overall diameter

overall_diameter (inches) = rim_diameter + 2 × (sidewall_height_mm / 25.4) For 205/55R16: 16 + 2 × (112.75 / 25.4) = 16 + 8.878 = 24.878 inches

Circumference

circumference = π × overall_diameter For 205/55R16: π × 24.878 = 78.16 inches

Revolutions per mile

rev/mile = 63,360 / circumference (in inches) For 205/55R16: 63,360 / 78.16 = 811 rev/mile

That 63,360 is not a magic number, it is just inches in a mile: 5,280 feet × 12.

Why this matters

Changing tire size changes your speedometer reading, because the speedometer and odometer are calibrated for one specific rolling circumference.
Going from 205/55R16 (24.88") to 225/50R16 (24.86") is a change of two hundredths of an inch, which nothing will ever notice. But going from 205/55R16 to 225/65R16 (27.52") adds 10.6% to the diameter, and the speedometer then reads 10.6% low: an indicated 60 mph is really 66.

The 60-series and 65-series tires common on trucks and SUVs are much taller than the 45-series and 50-series fitted to sports cars, which is why swapping a wheel package between the two rarely works out on diameter alone.

Section width in inches

section_width_inches = section_width_mm / 25.4 For 205mm: 205 / 25.4 = 8.07 inches