Enter your stock and new tire sizes to see diameter, circumference, and exactly how much your speedometer will read off after a tire change.
Typing a size here fills in the fields below automatically. You can also enter the three numbers by hand.
| Spec | Stock | New | Diff |
|---|
What the speedometer reads vs your actual speed with the new tires fitted (assumes the speedo was calibrated for the stock size).
Your speedometer measures speed by counting wheel revolutions per unit of time and multiplying by the tire's rolling circumference. Change the tire size and that circumference changes — so the speedo reads wrong until you correct it (via ECU calibration, aftermarket tuner, or a speedo healer).
A tire marked 225/45R17 means: 225 mm wide, 45% aspect ratio (sidewall height = 45% of 225 mm = 101.25 mm), fits a 17-inch wheel.
The correction factor is the ratio of new to old circumference. A factor above 1.00 means your speedometer reads low (you're going faster than it shows). Below 1.00 means it reads high.
All-wheel-drive systems are especially sensitive to circumference differences. When tires on the same vehicle rotate at different rates, the center differential, transfer case clutches, or viscous coupling must constantly slip to absorb the mismatch — generating heat and accelerating wear on components that cost thousands to replace. Most AWD manufacturers (Subaru, Audi quattro, BMW xDrive) specify that all four tires stay within roughly 1% of each other in rolling circumference; some are stricter, limiting tread-depth differences to as little as 2/32". The AWD Safe indicator above turns green when the two sizes are inside that ±1% window. If you're replacing fewer than four tires on an AWD car, mixing brands, or considering a staggered setup, check this number before anything else.
Switch the calculator to Front vs Rear mode to compare a staggered setup — wider, and often taller, tires on the drive axle. Half of the diameter difference is your ride-height change at that axle, which alters the car's rake: a taller rear tire lifts the tail and adds nose-down rake, slightly affecting aero balance and headlight aim. Two cautions with stagger: the tires can never be rotated front-to-rear, so they wear on their own schedules; and on AWD cars an aftermarket stagger that breaks the ±1% circumference rule will overwork the center differential — factory-staggered AWD cars (many BMW xDrive and Audi S/RS models) are engineered with front and rear circumferences deliberately matched even though the widths differ.
When upgrading to a larger wheel (e.g., 17→18 inch), the goal is to keep overall tire diameter the same by reducing aspect ratio. This maintains speedometer accuracy and preserves factory ride height and gear ratios. This calculator shows you how close you are to a matched diameter.
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