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.
Say your car came on 225/45R17 and you're eyeing 245/40R18 for a plus-one fitment. Work the stock diameter first:
Now the new size on the 18-inch wheel:
That's a diameter increase of about 0.75 inch, or roughly +3.0%. Plugging the circumferences into the speedo formula gives a correction factor of about 1.030, so at an indicated 60 mph you're actually traveling near 61.8 mph - the speedometer reads low. This sits right at the edge of the common 3% guideline: acceptable for most rear- or front-drive cars but worth a speedo recalibration if you want the odometer and cruise control exactly right, and over the ±1% limit for an AWD drivetrain unless every corner gets the same size. To stay closer to stock, a 245/35R18 (≈24.75 in) would land within about 1%.
Yes. The speedometer counts wheel revolutions and multiplies by the tire's rolling circumference. A taller tire makes the speedometer read low (you travel faster than indicated); a shorter tire makes it read high. Differences beyond about 3% warrant recalibration.
225 is the section width in millimeters. 45 is the aspect ratio, meaning the sidewall height is 45% of the width (here 101.25 mm). The R means radial construction, and 17 is the wheel diameter in inches that the tire fits. Overall diameter is the wheel diameter plus twice the sidewall height.
A widely used rule of thumb is to keep overall diameter within plus or minus 3% of the original. Within that window the speedometer error, gearing change, and ABS/traction-control calibration drift stay small. Larger changes can affect speedometer accuracy, fuel-gauge range estimates, and on some cars trigger warning lights.
Yes. A taller tire travels farther per revolution, so it effectively lengthens (numerically lowers) your gearing. The engine turns fewer RPM at a given road speed, which can dull off-the-line response. A shorter tire does the opposite, raising RPM and sharpening acceleration while lowering top speed.
Most AWD manufacturers specify all four tires stay within roughly 1% of each other in rolling circumference - some limit tread depth differences to as little as 2/32 inch. Larger mismatches force the center differential or transfer case clutches to slip constantly, causing expensive drivetrain damage.
Related Calculators