A tire's sidewall size code — like 225/60R16 — packs three numbers that together determine its overall mounted diameter, and from that, its rolling circumference and how many times it turns per mile. This calculator decodes the size code and runs the math instantly.
Reading a sidewall size code
A metric tire size such as 225/60R16 reads as: 225mm section width, 60% aspect ratio, radial construction, mounted on a 16-inch wheel. The section width and aspect ratio together determine the sidewall height, which is added twice (top and bottom of the wheel) to the wheel diameter to get the tire's overall mounted diameter.
From diameter to circumference and revolutions per mile
Once the overall diameter is known, the rolling circumference follows directly as π x diameter — the distance the tire covers in one full turn. Dividing the number of inches in a mile (63,360) by that circumference gives revolutions per mile, the figure that odometers, speedometers, and differential gearing calculations are all built on.
Why this matters for plus-sizing and tire swaps
Changing tire size — whether plus-sizing to a larger wheel with a shorter sidewall, or simply replacing a worn tire with a different brand's slightly different size — changes the overall diameter and therefore the revolutions per mile. A taller replacement tire turns fewer times per mile than the stock size and makes the speedometer and odometer under-read; a shorter one makes them over-read. Keeping the overall diameter within a few percent of the original avoids meaningfully changing your gearing, fuel economy readings, or cruise control accuracy.