Tyre size calculator
Compare your current tyre size with another: overall diameter, sidewall height, width, circumference, revolutions per kilometre and what your speedometer will show. A size more than 3% off the original’s diameter is flagged. The worked example compares 185/65R15 and 195/55R16.
| Measure | 165/55R14 | 195/55R16 | Difference |
|---|---|---|---|
| Section width | 165 mm | 195 mm | +30.0 mm |
| Sidewall height | 90.8 mm | 107.3 mm | +16.5 mm |
| Overall diameter | 537.1 mm | 620.9 mm | +83.8 mm |
| Circumference | 1687.3 mm | 1950.6 mm | +263.3 mm |
| Revolutions per km | 592.6 | 512.7 | −80.0 |
| Diameter difference | +15.60% |
195/55R16 is 15.6% taller overall than 165/55R14 (+83.8 mm): outside the common ±3% tolerance. Check with a fitter before switching.
What the speedometer shows on 195/55R16
For a car set up for 165/55R14. The speedometer reads 13.5% low: you are going faster than it shows.
| Speedometer shows | Actual speed on 195/55R16 | Difference |
|---|---|---|
| 60 km/h | 69.4 km/h | +9.4 km/h |
| 100 km/h | 115.6 km/h | +15.6 km/h |
| 120 km/h | 138.7 km/h | +18.7 km/h |
Listed sizes within 3% of 165/55R14
Sizes dealers list on CircleWheels whose overall diameter is within 3% of 165/55R14’s. Compare one above, then check the fitment with a fitter.
How to read a tyre size
Take 185/65R15. 185 is the section width: how wide the tyre is from sidewall to sidewall, in millimetres. 65 is the aspect ratio, or profile: the sidewall’s height as a percentage of that width, so 65% of 185 mm gives a sidewall of 120.25 mm. R means radial construction, which nearly every car tyre has. 15 is the diameter of the wheel rim it fits, in inches.
The overall diameter is the rim plus two sidewalls: 15 inches is 381 mm, and 381 + 2 × 120.25 = 621.5 mm. The circumference is the diameter times π, about 1,952 mm, and one kilometre divided by the circumference gives the revolutions per kilometre, about 512.
These are the dimensions the size itself states, for a tyre off the car. A tyre carrying a car’s weight rolls on a slightly smaller radius, and two makers’ tyres of the same size can measure a few millimetres apart.
Why 3%
A speedometer and an odometer count how often the wheels turn, not how far the car goes. A taller tyre covers more ground with each turn, so the car goes faster than the speedometer shows and further than the odometer records; a shorter tyre does the opposite. The speed table above shows by how much.
Fitters commonly keep a replacement within about 3% of the original overall diameter. It is a rule of thumb, not a law. Beyond it the speedometer error grows, the gearing changes, and a taller or wider tyre is more likely to rub on the wheel arches or the suspension.
Speedometers are usually set to read a little high: UN Regulation 39, followed in Europe, Japan and many other markets, says a speedometer must never show less than the true speed. A taller tyre uses up some or all of that margin.
Before you change size
Diameter is one check of several. The new size also has to suit the width of your wheel rims, clear the bodywork at full steering lock and over bumps, and carry at least the load index and speed rating your car’s tyre placard gives. This calculator doesn’t check those: the placard, usually on the driver’s door frame, and a fitter can.