Tyre Size Comparison Calculator

What this generator does

Turns each marking into a real diameter — rim in inches plus two sidewalls in millimetres — and compares the two, including what a speedometer calibrated for the first will read on the second.

How to use this tool

1. Type the size on the car now, off the sidewall.
2. Type the size you are considering.
3. Read the difference in diameter, and what the speedometer will show.
4. Anything beyond three per cent is outside what the trade usually accepts.

Understanding the controls

The tyre on the car now

As written on the sidewall: tread width in millimetres, the aspect ratio, then the rim in inches. The letter between them can be R, ZR or left out.

The tyre you are thinking of

The size you are comparing against. The comparison is one-way — the first is treated as what the speedometer was calibrated for.

Common use cases

• Checking whether a cheaper size will do instead of the one fitted
• Working out speedometer error after a change of wheels
• Plus sizing to a larger rim while keeping the overall diameter
• Seeing how much higher or lower the car will sit
• Comparing winter tyres on smaller rims against the summer set

How this generator works

The overall diameter is the rim converted to millimetres plus twice the sidewall, and the sidewall is the aspect ratio applied to the tread width. Rolling circumference follows, and from that the turns of the wheel per kilometre — which is what a speedometer actually counts, and why a taller tyre makes the dial read low. The checks invert the arithmetic rather than repeating it: the aspect ratio is recovered from the finished diameter and compared with the marking it came from, and the speed conversion is run backwards, so an indicated hundred converted to a true speed and back must return exactly a hundred. The ride height change is checked to be half the diameter change, since only the bottom half of the tyre sits between the wheel and the road.

Randomness and fairness

Nothing is random. The same two sizes always compare the same way.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

• This is geometry, not fitment: whether a tyre clears the arches and suspension depends on the wheel offset and width, which markings do not tell you.
• Load and speed ratings matter and are not compared here.
• Real rolling circumference is slightly less than the geometric figure, because a loaded tyre squashes.
• Speedometers are required to read high rather than low, so the dial was never exactly right to begin with.

Privacy and your data

The comparison is worked out in your browser. No sizes are transmitted or stored.