Tyre Size Comparison Calculator
Compare two tyre sizes and see the speedometer difference.
What is the Tyre Size Comparison Calculator?
A tyre size comparison calculator compares two tyre sizes (given as width/aspect ratio/rim diameter, e.g. 205/55R16) to show the difference in overall rolling diameter and how that would affect speedometer accuracy. It works by calculating each tyre's overall diameter — rim diameter converted to millimetres, plus twice the sidewall height (tyre width × aspect ratio ÷ 100) — then comparing the two diameters as a percentage difference and scaling a reference speed to show the speedometer effect. To calculate it manually: diameter = (rim × 25.4) + (2 × width × aspect ÷ 100) for each tyre, then percentage difference = (new − old) ÷ old × 100 — a significant diameter change affects gearing, ABS/ESP calibration and legal compliance, so stay within your tyre manufacturer's and vehicle handbook's approved size range.
How it works
A tyre size code (like 205/55R16) encodes the tyre's width in millimetres, its aspect ratio (sidewall height as a percentage of width), and the rim diameter in inches. From these three numbers, the tyre's overall diameter can be calculated directly, which lets you compare how two different tyre sizes actually measure up against each other.
UK context
Changing to a tyre size with a meaningfully different overall diameter than the vehicle manufacturer's specification can throw off your speedometer accuracy (since it's calibrated for the original wheel rotation rate), and can also affect ABS/ESP calibration and, in some cases, insurance or legal compliance depending on how far outside the approved range you go.
Tips
- Always check your vehicle manufacturer's approved tyre size range before fitting a non-standard size — this calculator shows the geometric effect, not whether a specific size is actually approved for your car.
- A small size difference (a percent or two in diameter) is generally considered acceptable and common practice when tyre shops recommend an alternative size for cost or availability, but larger differences can cause real issues.
- Changing overall diameter also changes your effective gearing slightly, which can have a small effect on acceleration and fuel economy, not just the speedometer reading.
Frequently asked questions
How is tyre diameter calculated from the size code?
Overall diameter equals the rim diameter (converted from inches to millimetres) plus twice the sidewall height, where sidewall height is the tyre's width multiplied by its aspect ratio as a percentage.
Why does changing tyre size affect my speedometer?
Your speedometer is calibrated based on wheel rotation rate for the original tyre diameter — if the new tyre has a different rolling circumference, the same rotation rate now corresponds to a different actual road speed, throwing off the reading.
Is it legal to fit a different tyre size than standard?
It can be, within your vehicle manufacturer's approved range and provided the tyres are otherwise roadworthy and correctly rated — but going outside the approved range can create legal, safety and insurance issues. Always check manufacturer guidance.
A quick note
This shows the geometric effect of a tyre size change based on the numbers entered — it doesn't confirm whether a specific size is approved for your vehicle. Always check your manufacturer's approved tyre size range.