Enter your original and new tire sizes to see the diameter change, your actual speed at common indicated speeds, and how far the odometer will be off.
The same percentage applies to the speedometer and, on most vehicles, the odometer, because they share a speed signal. Real tires vary slightly by brand, inflation and tread depth.
| Original diameter | – |
|---|---|
| New diameter | – |
| Diameter change | – |
| After 100 indicated miles | – |
| Odometer after 100 real miles | – |
| Indicated | Actual speed | Error |
|---|
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Overall diameter = rim (inches) × 25.4 + 2 × section width × (aspect ratio ÷ 100). Actual speed = indicated speed × (new diameter ÷ original diameter). A 225/45R17 is 24.97 inches across from the size code. Going to a taller tire makes the gauge read low; a shorter tire makes it read high.
Compare full size, sidewall and revolutions per mile with the tire size calculator, or see how RPM changes with the gear ratio & RPM calculator.
Many shops aim to stay within about 3% of stock diameter so speedometer, odometer and wheel-speed systems stay close to design. It is not a hard legal limit, and a change inside 3% can still be a poor fit if the tire is much wider or the wheel offset is aggressive.
A taller tire covers more ground per revolution. The car still counts the same number of wheel turns, so the gauge thinks you are going slower than you really are. At an indicated 60 mph, actual speed is 60 × (new diameter ÷ old diameter).
Usually yes, because the speedometer and odometer typically share the same vehicle-speed signal. If the tire is 3% taller, about 103 real miles pass for every 100 miles the odometer shows.
Many shops try to keep overall diameter within about 3% of stock to limit speedometer, odometer, ABS and stability-control issues. It is a rule of thumb, not a legal limit or a guarantee that a change under 3% is fine on every car.
It can. Those systems infer wheel speed from the same (or similar) sensors. A large diameter change, or a big size mismatch from front to rear or left to right, can confuse them. Stay close to stock and keep axle sizes consistent unless the vehicle was designed for a stagger.
If the diameter change is more than about 3%, or if you care about indicated speed and recorded mileage, a recalibration (or a handheld programmer on vehicles that support it) is worth asking a shop about. Some cars can be coded with the new size; others need a different speedo gear or module.
They come from the size code: overall diameter = rim (inches) × 25.4 + 2 × section width × aspect ratio. Real tires vary a little by brand, load, inflation and tread depth. A worn tire is slightly shorter than a new one of the same size.