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 can be sensitive to front-to-rear rolling differences, but there is no universal percentage that makes a tire combination “safe.” Manufacturers may specify mounted circumference, tread-depth difference, approved staggered pairs, or another vehicle-specific limit. The screening result above calls a nominal difference within 1% a close match; it is not an approval. If you are replacing fewer than four tires, mixing models, or planning a staggered setup, compare actual mounted circumference and tread depth with the owner's or service manual.
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. Staggered tires cannot be rotated front-to-rear, and an AWD stagger must match a combination approved for that exact vehicle. Nominal sizes alone do not capture mounted circumference, tire-model variation, pressure, load, or tread wear.
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. That is a meaningful change to verify for clearance and calibration. On AWD vehicles, compare the proposed combination with the exact manufacturer specification rather than treating a generic percentage as approval. A 245/35R18 (≈24.75 in) is a closer nominal match.
The formula follows the nominal tire-size convention. Mounted dimensions vary with measuring rim, pressure, load, tread, and manufacturer tolerance. Review NHTSA tire-safety guidance, the tire maker's specification sheet, and the vehicle manual before treating a calculated size as approved fitment.
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.
There is no universal approved percentage. Use the nominal difference to understand speedometer and gearing effects, then verify clearance, ratings, wheel compatibility, control systems, and AWD requirements for the exact vehicle.
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.
There is no universal AWD limit. The allowable difference can be stated as circumference, diameter, tread depth, or a vehicle-specific front-to-rear pairing. Use this page to screen the nominal size difference, then follow the exact tire requirements in the vehicle owner's or service manual.
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