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
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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