Tire Size Calculator
Compare two tire sizes and see the diameter change, real speed and revolutions per mile.
Changing tire size changes more than looks. A taller tire makes the speedometer read low, a shorter one makes it read high, and either can affect gearing, clearance and ride. Enter your current size and the size you are considering to see the difference at once.
How the tire size calculator works
A tire size such as 225/65R17 holds three numbers: 225 is the section width in millimetres, 65 is the sidewall height as a percentage of that width (the aspect ratio), and 17 is the wheel diameter in inches. The calculator turns them into an overall diameter.
overall diameter (mm) = rim diameter × 25.4 + 2 × width × aspect ratio ÷ 100.
Divide by 25.4 for inches.
Worked example: 225/65R17 to 245/65R17
- Current tire: 17 × 25.4 + 2 × 225 × 0.65 = 724.3 mm, or 28.5 inches.
- New tire: 17 × 25.4 + 2 × 245 × 0.65 = 750.3 mm, or 29.5 inches.
- Difference: 750.3 ÷ 724.3 = 1.036, so the new tire is about 3.6% taller.
- When the speedometer shows 60 mph, the real speed is about 60 × 1.036 = 62.2 mph.
- Revolutions per mile fall from about 707 to about 683, so the engine turns slightly slower at the same road speed.
Why the speedometer reading changes
The speedometer counts wheel rotations and assumes the original tire size. A larger tire covers more ground per rotation, so the car is really faster than the dial shows. A smaller tire does the opposite. The same error affects the odometer, so trip distance and service intervals shift too.
Common mistakes
- Changing only the width and forgetting that the sidewall height scales with it.
- Ignoring the rim size when going up a wheel size. A bigger wheel needs a lower aspect ratio to keep the same overall diameter.
- Skipping the load index and speed rating on the sidewall. They must meet or exceed the vehicle maker’s rating.
- Not checking clearance at full steering lock and over bumps, especially for wider tires.
- Treating a calculator as a fitment guarantee. Always confirm with the owner manual or a tire shop.
| Change | Effect on speedometer | Effect on gearing |
|---|---|---|
| Taller tire | Reads low (real speed higher) | Feels taller geared, slower acceleration |
| Shorter tire | Reads high (real speed lower) | Feels shorter geared, quicker acceleration |
| Same diameter | No meaningful change | No meaningful change |
Frequently asked questions
What does 225/65R17 mean?
225 is the tire width in millimetres, 65 is the sidewall height as a percentage of the width, R means radial construction and 17 is the wheel diameter in inches.
How much tire size difference is acceptable?
A common rule of thumb is to stay within about 3% of the original overall diameter. Check your owner manual and a tire professional for your vehicle.
Does a bigger tire hurt fuel economy?
It can. Larger and wider tires are usually heavier and have more rolling resistance, and the taller overall diameter changes the effective gearing.
Will the speedometer be wrong after changing tires?
Yes if the overall diameter changes. A taller tire makes the speedometer read lower than your real speed.