Converters
Turn a metric tire code like 225/65R17 into real dimensions — overall diameter, sidewall height, circumference, revolutions per mile — and compare two sizes side by side.
A metric size like 225/65R17 reads as: 225 mm section width, an aspect ratio of 65 (the sidewall is 65% as tall as the tire is wide), R for radial construction, on a 17-inch wheel. Sidewall height = width × aspect ratio ÷ 100 (in mm, ÷ 25.4 for inches). Overall diameter = wheel diameter + two sidewalls. Circumference = π × diameter, and revolutions per mile = 63,360 inches in a mile ÷ circumference in inches.
Speedometer error: your speedometer counts wheel revolutions and assumes the stock tire's circumference. Fit a tire with a larger diameter and each revolution carries you farther than the car thinks — when the speedometer reads 60 mph you are actually going 60 × (new diameter ÷ old diameter). The odometer and trip computer drift by the same percentage. As a general rule of thumb, staying within about 3% of the stock overall diameter keeps speedometer error and gearing effects small; this tool shows the exact figure from the size codes.
Worked example: 225/65R17 — sidewall 225 × 0.65 = 146.3 mm (5.76 in), diameter 17 + 2 × 5.76 = 28.5 in, circumference 89.6 in, and 63,360 ÷ 89.6 ≈ 707 revolutions per mile. These are nominal figures from the size code; a specific tire's measured diameter varies a little by brand, load, and wear.
What does the "R" mean? Radial construction — nearly every modern passenger tire. It carries no number, so it is not an input here; the three numbers are width, aspect ratio, and wheel diameter.
Can I compare a taller tire for a truck? Yes — enter both codes and read the diameter difference and speedometer effect. Remember the comparison is diameter only: width, load rating, and clearance against the wheel well and suspension are separate questions this calculator does not answer.
Why doesn't my measured diameter match exactly? The size code is nominal. Tread depth, inflation, load, and each manufacturer's rounding move the real rolling diameter by a few tenths of an inch, which is why revs-per-mile figures published by tire makers differ slightly from the pure code math.
More tools — Converters
Cooking Converter · Oven Temperature Converter · Ring Size Converter · Shoe Size Converter · Fuel Economy Calculator · Full index
Every tool on this site runs in your browser. Nothing you type is sent anywhere, stored, or tracked.