Tire Size Conversion Calculator
Convert metric tire sizes (205/55R16) to sidewall height, overall diameter, circumference, and revolutions per mile.
Works for any standard tire code.
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 = 78.13 inches
Revolutions per mile
rev/mile = 63,360 / circumference (in inches) For 205/55R16: 63,360 / 78.13 = 811 rev/mile
Why this matters
Changing tire size changes your speedometer reading because the odometer and speedometer are calibrated for a specific tire circumference. Going from 205/55R16 (24.88") to 225/50R16 (24.84") is nearly identical in diameter. But going from 205/55R16 to 225/65R16 (27.52") increases diameter by 10.6%, making your speedometer read 10.6% low.
The 60-series and 65-series tires are common on trucks and SUVs and are significantly taller than the 45-series and 50-series performance tires on sports cars.
Section width in inches
section_width_inches = section_width_mm / 25.4 For 205mm: 205 / 25.4 = 8.07 inches