Tire Size Calculator
Compare two tyre sizes and find the speedometer error from changing them.
What the Tire Size Calculator does
A tyre size code mixes units: the section width is in millimetres, the aspect ratio is a percentage of that width, and the rim diameter is in inches. Overall diameter is the rim plus two sidewalls, and it is that figure — not the width — which determines speedometer accuracy.
Formula
Sidewall height = Section width × Aspect ratio ÷ 100Overall diameter = Rim diameter + 2 × Sidewall heightActual speed = Indicated × New diameter ÷ Original diameterRevolutions per mile = 63,360 ÷ (π × Diameter)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Original tyre size | text | Yes | Format: 225/45R17 |
| New tyre size | text | Yes | Format: 245/40R18 |
| Indicated speed | number | Optional | What the speedometer reads on the new tyres. Accepts 0 or more. |
| Distance unit | one of 2 options | Yes | — |
How to use it
- Choose Distance unit.
- Enter Original tyre size and New tyre size.
- Optionally add Indicated speed.
- Select Calculate.
Worked example
Changing from 225/45R17 to 245/40R18.
- Original
- 225/45R17
- New
- 245/40R18
- Indicated
- 60 mph
Diameter goes from 24.97 in to 25.72 in, a 3.0% increase. At an indicated 60 mph you are really doing 61.8 mph.
Frequently asked questions
How much difference in tyre size is safe?
Within about 3% of overall diameter. Beyond that the speedometer, ABS and traction control all work from figures that no longer match reality.
What does 225/45R17 actually mean?
225 mm section width, sidewall height 45% of that width, radial construction, fitted to a 17 inch rim.