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Tire Size Calculator
Enter two tire sizes to see diameter, circumference and the likely speedometer error.
Where does a tire's diameter come from?
A tire's sidewall shows a size like 225/45R17, made of three parts: the tire's width in millimeters (225), the aspect ratio — sidewall height as a percent of width (45%) — and the rim diameter in inches (17). From these three numbers you can calculate the tire's overall diameter: the sidewall height equals width times aspect ratio divided by 100; double that (since it covers both above and below the rim) and add the rim diameter (converted from inches to millimeters by multiplying by 25.4). The tire's circumference equals overall diameter times pi.
These numbers matter in practice: a car's speedometer and odometer are calibrated based on wheel revolutions per unit time and the original tire's diameter. If a replacement tire has a different diameter, the rolling circumference changes too — the same number of revolutions covers a different actual distance, so the displayed speed and recorded mileage drift slightly from reality. That's why, when changing tire sizes, it's best to keep the diameter difference under about 3%.
Frequently asked questions
Why does a tire size difference affect the speedometer?
The speedometer works off wheel revolutions per unit time and the tire diameter the manufacturer calibrated it for. If a new tire has a different diameter, the same number of wheel revolutions covers a different actual distance, so the speedometer and odometer read slightly off.
How much tire diameter difference is acceptable?
A common industry rule of thumb is to keep the overall diameter difference between two tire sizes under about 3%, so the speedometer, ABS and stability control keep working correctly.
Why should all four tires be the same size?
A diameter mismatch between wheels (front vs. rear, for example) can confuse the differential, ABS and traction control systems, since they assume every wheel is rotating at a consistent rate relative to its size.
How do I read a tire size off the sidewall?
A size like 225/45R17 breaks down as: the first number is the tire's width in millimeters, the second is the aspect ratio (sidewall height as a percent of width), and the number after the R is the rim diameter in inches.