Compare two tyre sizes and see how the overall diameter and your speedometer reading change. Enter the width, aspect ratio and rim size printed on the sidewall of your old tyre and of the new one. The calculator shows the sidewall height, overall diameter and circumference of each, the difference between them, and the real speed for a speedometer reading you enter.
You read the numbers from your own tyres. This tool does not suggest a size for any vehicle.
Old tyre (the size your speedometer is set for)
New tyre
Read the three numbers from the sidewall of each tyre. A code such as 195/65 R15 means width 195 mm, aspect ratio 65%, rim 15 inches. This tool does not suggest any size.
Nothing you type is stored or sent anywhere. Results are calculated from the numbers you enter.
How to read the tyre size on the sidewall
A tyre size is printed as a code such as 195/65 R15 (an example only). The first number is the tyre width in millimetres, the second is the aspect ratio (the sidewall height as a percentage of the width), and the number after R is the rim diameter in inches. Other letters and numbers on the sidewall, such as load and speed ratings, are not used by this calculator.
How to use the tyre size calculator
- Enter the width, aspect ratio and rim diameter of the old tyre, the size your speedometer is set for.
- Enter the same three numbers for the new tyre.
- Optionally enter the speed your speedometer shows.
- Press “Compare tyre sizes”.
The formulas
- Sidewall height = width × aspect ratio ÷ 100
- Overall diameter = rim diameter × 25.4 + 2 × sidewall height (in mm)
- Circumference = π × overall diameter
- Difference = new overall diameter − old overall diameter, also shown as a percentage of the old diameter
- Real speed = speedometer reading × new diameter ÷ old diameter
What this calculator does not include
- Whether a tyre size fits your vehicle, rim or wheel arch. Check your vehicle manual and ask a tyre dealer.
- Load rating, speed rating, tyre pressure or tyre condition.
- Any legal limit on tyre sizes. The note shown for a difference of 3% or more is our own caution, not a rule.
- Exact figures for a tyre in use. Real diameter changes with wear, pressure, load and speed, and makers’ measured sizes can differ slightly from the formula.
More tools for vehicle owners
See what your vehicle costs to run with the vehicle ownership cost calculator and plan your next service with the vehicle service reminder calculator. Replacing your tyres? Work out their running cost with the tyre cost per km calculator.
Tyre size calculator FAQs
How do I find my tyre size?
It is printed on the sidewall of the tyre, in a code such as 195/65 R15. Your vehicle manual may also list it.
Why does my speedometer read differently with a new tyre size?
The speedometer counts wheel turns. A tyre with a different overall diameter covers a different distance per turn, so the real speed differs from what the dial shows.
Can I fit any size I like?
This tool cannot tell you that. Ask your tyre dealer and check your vehicle manual for sizes that suit your vehicle.
Is my data stored?
No. The calculation runs in your browser, and nothing you enter is sent to our servers.