Tire calculator
Calculate rotation speed and tire size
Compare two tire sizes and see what they do to rotation speed, diameter, circumference and the speedometer.
Find the car and get the tire size it left the factory on.
The original size is what the model shipped with. A car may be approved for several sizes - check the registration papers.
Suggested alternative sizes
Sizes that roll within ±3% of your current tires. Pick one to run the numbers on it.
14 inches
Two inches down
15 inches
One inch down
16 inches
Same rim
17 inches
One inch up
18 inches
Two inches up
Before you choose
- Check the registration papers. Only the sizes the car is approved for may be fitted - the tolerance here is a starting point, not permission.
- Keep at least the same load index and speed rating as the original tire. Those are the characters after the size, e.g. 91V.
- Run the same size on both sides of an axle, and ideally change all four at once.
- Wider tires and lower profiles take up more room in the arch. Check for contact at full lock and full load.
- For winter tires, one inch down and a narrower size is often the better buy - more rubber on the road and better grip in snow.
Rotation speed
How fast the wheel turns at a given road speed. A larger tire covers more ground per revolution, so it turns more slowly.
- Current · 205/55 R16
- 924rpm
503.7 revolutions per km
- New · 225/45 R17
- 920rpm
501.8 revolutions per km
| Speed | Current | New | Difference |
|---|---|---|---|
| 50 km/h | 420 rpm | 418 rpm | -2 |
| 80 km/h | 672 rpm | 669 rpm | -3 |
| 110 km/h | 924 rpm | 920 rpm | -3 |
| 130 km/h | 1,091 rpm | 1,087 rpm | -4 |
Measurements and difference
How the two sizes differ physically.
| Measurement | 205/55 R16 | 225/45 R17 | Difference |
|---|---|---|---|
| Diameter | 631.9 mm | 634.3 mm | +2.4 mm (+0.38%) |
| Circumference | 1,985 mm | 1,993 mm | +8 mm (+0.38%) |
| Sidewall height | 112.8 mm | 101.3 mm | -11.5 mm |
| Revolutions per km | 503.7 | 501.8 | -1.9 |
| Ride height | — | — | +1.2 mm |
The tire grows evenly all the way round, so the car rises by half the change in diameter.
±3% is a rule of thumb, not an approval. Your registration papers and the car's type approval decide which sizes you may actually fit.
Speedometer error
The speedometer is calibrated for the current tire. Change the size and it no longer shows your real speed.
Speedometer reads
50 km/h
Actual speed
50.2km/h
Error +0.2 km/h
Speedometer reads
80 km/h
Actual speed
80.3km/h
Error +0.3 km/h
Speedometer reads
110 km/h
Actual speed
110.4km/h
Error +0.4 km/h
Speedometer reads
130 km/h
Actual speed
130.5km/h
Error +0.5 km/h
This assumes the speedometer is accurate on the current size. Most speedometers already read slightly high from the factory.
Load and speed index
The characters after the size. In 205/55 R16 91V, the 91 is how much the tire may carry and the V is how fast it may run.
Enter or look up a size that carries its index - 205/55 R16 91V, say - and we will mark your values in the tables.
Load index
Maximum weight per tire at the correct pressure.
| Index | Max load |
|---|---|
| 60 | 250 kg |
| 61 | 257 kg |
| 62 | 265 kg |
| 63 | 272 kg |
| 64 | 280 kg |
| 65 | 290 kg |
| 66 | 300 kg |
| 67 | 307 kg |
| 68 | 315 kg |
| 69 | 325 kg |
| 70 | 335 kg |
| 71 | 345 kg |
| 72 | 355 kg |
| 73 | 365 kg |
| 74 | 375 kg |
| 75 | 387 kg |
| 76 | 400 kg |
| 77 | 412 kg |
| 78 | 425 kg |
| 79 | 437 kg |
| 80 | 450 kg |
| 81 | 462 kg |
| 82 | 475 kg |
| 83 | 487 kg |
| Index | Max load |
|---|---|
| 84 | 500 kg |
| 85 | 515 kg |
| 86 | 530 kg |
| 87 | 545 kg |
| 88 | 560 kg |
| 89 | 580 kg |
| 90 | 600 kg |
| 91 | 615 kg |
| 92 | 630 kg |
| 93 | 650 kg |
| 94 | 670 kg |
| 95 | 690 kg |
| 96 | 710 kg |
| 97 | 730 kg |
| 98 | 750 kg |
| 99 | 775 kg |
| 100 | 800 kg |
| 101 | 825 kg |
| 102 | 850 kg |
| 103 | 875 kg |
| 104 | 900 kg |
| 105 | 925 kg |
| 106 | 950 kg |
| 107 | 975 kg |
| Index | Max load |
|---|---|
| 108 | 1,000 kg |
| 109 | 1,030 kg |
| 110 | 1,060 kg |
| 111 | 1,090 kg |
| 112 | 1,120 kg |
| 113 | 1,150 kg |
| 114 | 1,180 kg |
| 115 | 1,215 kg |
| 116 | 1,250 kg |
| 117 | 1,285 kg |
| 118 | 1,320 kg |
| 119 | 1,360 kg |
| 120 | 1,400 kg |
| 121 | 1,450 kg |
| 122 | 1,500 kg |
| 123 | 1,550 kg |
| 124 | 1,600 kg |
| 125 | 1,650 kg |
| 126 | 1,700 kg |
| 127 | 1,750 kg |
| 128 | 1,800 kg |
| 129 | 1,850 kg |
| 130 | 1,900 kg |
Speed index
The highest speed the tire is approved for at full load.
| Index | Max speed |
|---|---|
| L | 120 km/h |
| M | 130 km/h |
| N | 140 km/h |
| P | 150 km/h |
| Q | 160 km/h |
| R | 170 km/h |
| S | 180 km/h |
| T | 190 km/h |
| U | 200 km/h |
| H | 210 km/h |
| V | 240 km/h |
| ZR | over 240 km/h |
| W | 270 km/h |
| Y | 300 km/h |
ZR is an older marking with no ceiling of its own. Tires carrying it also print a real rating after the size.
Always fit at least the same index as the original tire. Going lower is not legal and your insurer may refuse a claim. Higher is fine - though winter tires may run a lower speed index if you put a sticker in the cab.
How we calculate it
How we calculate it
Every number comes from the code on the tire's sidewall.
The tire code
In 205/55 R16, 205 is the tire's width in millimetres, 55 is the aspect ratio - the sidewall height as a percentage of the width - and 16 is the rim diameter in inches. The sidewall is 205 × 55% = 112.75 mm, and it counts both above and below the rim.
Diameter and circumference
The diameter is the rim in millimetres plus two sidewalls, and the circumference is the diameter times pi. The circumference is the distance the car travels per wheel revolution, which is where both rotation speed and the speedometer start.
Rotation speed
At 100 km/h the car covers 1,666,667 mm per minute. Divide by the tire's circumference and you have its rotation speed. A larger tire turns fewer times at the same speed, which is also why the engine settles at slightly lower revs.
Speedometer and odometer
The speedometer counts wheel revolutions and multiplies by the circumference it was calibrated for. Fit a larger tire and you travel further per revolution than the car assumes, so both your speed and your mileage read low.