Tire size calculator and comparison
Enter your current tire size and the new one exactly as printed on the sidewall (e.g. 205/55R16 91V). The calculator shows overall diameter, circumference and sidewall height of both tires in millimeters and inches, the difference in percent and how fast you are really going when the speedometer shows a given speed.
How it is calculated
How to read a tire size
Take 205/55R16 91V: 205 is the section width in millimeters, 55 the aspect ratio (the sidewall is 55 % as tall as the tire is wide), R means radial construction and 16 is the rim diameter in inches. 91 is the load index and V the speed rating. A “P” in front (P215/65R15) marks a P-metric passenger tire, “ZR” instead of “R” a radial tire rated above 240 km/h (149 mph), speed rating W or Y, under UN Regulation No. 30.
How to compare tire sizes
- Enter your current tire as printed on the sidewall, e.g. 205/55 R16 91V.
- Enter the new tire in the same format.
- Choose the speedometer unit and enter what the speedometer shows.
- Compare diameter, circumference and sidewall height, the size difference in percent and your actual speed.
How the tire size calculator works
UN Regulation No. 30 (para. 6.1.2.1) gives the same formula ETRTO uses:
- Sidewall height H = width × aspect ratio ÷ 100
- Rim diameter d = inches × 25.4 mm
- Overall diameter D = d + 2 × H
- Circumference = π × D
Example 205/55R16: H = 205 × 55 ÷ 100 = 112.75 mm; d = 16 × 25.4 = 406.4 mm; D = 406.4 + 2 × 112.75 = 631.9 mm (24.88 in); circumference = π × 631.9 ≈ 1,985 mm (78.2 in). These are nominal sizes of a new tire; the effective rolling circumference under load is slightly smaller and changes with tread wear and pressure.
Speedometer error with different tire sizes
The speedometer counts wheel revolutions and assumes the circumference of the tire it was calibrated for. So actual speed = speedometer reading × new circumference ÷ old circumference. A taller tire covers more ground per revolution, so you drive faster than the speedometer shows – and the odometer counts fewer miles than you drive.
Example: from 205/55R16 (631.9 mm) to 225/45R17 (431.8 + 2 × 101.25 = 634.3 mm): 634.3 ÷ 631.9 = 1.0038, i.e. +0.38 %. At 60 mph on the speedometer you are doing 60.2 mph. The other way, from 195/65R15 (634.5 mm) to 185/60R15 (603.0 mm): −4.96 %, so 80 km/h shown means just 76.0 km/h.
Under UN Regulation No. 39 (para. 5.4), the international type-approval rule for speedometers, a speedometer must never show less than the true speed and at most 10 % + 4 km/h more (0 ≤ Vshown − Vtrue ≤ 0.1 × Vtrue + 4 km/h). That is why many speedometers read slightly high from the factory; a taller tire eats into that margin. How much margin your car has can only be measured (e.g. with GPS), not derived from the tire size.
Which tire sizes are allowed?
The sizes approved for your car are listed on the tire placard (usually on the driver’s door jamb) and in the owner’s manual. Any other size should keep at least the original load index and speed rating; rules on plus-sizing differ by country, so check with your tire dealer or the inspection authority where you live.
Load index chart (capacity per tire)
From UN Regulation No. 30, Annex 4. The capacity applies to each tire: load index 91 = 615 kg (1,356 lb), so four tires carry up to 2,460 kg.
| LI | kg | LI | kg | LI | kg |
|---|---|---|---|---|---|
| 60 | 250 | 81 | 462 | 102 | 850 |
| 61 | 257 | 82 | 475 | 103 | 875 |
| 62 | 265 | 83 | 487 | 104 | 900 |
| 63 | 272 | 84 | 500 | 105 | 925 |
| 64 | 280 | 85 | 515 | 106 | 950 |
| 65 | 290 | 86 | 530 | 107 | 975 |
| 66 | 300 | 87 | 545 | 108 | 1,000 |
| 67 | 307 | 88 | 560 | 109 | 1,030 |
| 68 | 315 | 89 | 580 | 110 | 1,060 |
| 69 | 325 | 90 | 600 | 111 | 1,090 |
| 70 | 335 | 91 | 615 | 112 | 1,120 |
| 71 | 345 | 92 | 630 | 113 | 1,150 |
| 72 | 355 | 93 | 650 | 114 | 1,180 |
| 73 | 365 | 94 | 670 | 115 | 1,215 |
| 74 | 375 | 95 | 690 | 116 | 1,250 |
| 75 | 387 | 96 | 710 | 117 | 1,285 |
| 76 | 400 | 97 | 730 | 118 | 1,320 |
| 77 | 412 | 98 | 750 | 119 | 1,360 |
| 78 | 425 | 99 | 775 | 120 | 1,400 |
| 79 | 437 | 100 | 800 | ||
| 80 | 450 | 101 | 825 |
Speed rating chart
From UN Regulation No. 30, para. 2.29.1 – the maximum speed the tire is designed for (mph rounded).
| Symbol | max. km/h | max. mph |
|---|---|---|
| L | 120 | 75 |
| M | 130 | 81 |
| N | 140 | 87 |
| P | 150 | 93 |
| Q | 160 | 99 |
| R | 170 | 106 |
| S | 180 | 112 |
| T | 190 | 118 |
| U | 200 | 124 |
| H | 210 | 130 |
| V | 240 | 149 |
| W | 270 | 168 |
| Y | 300 | 186 |
More for drivers: the stopping distance calculator, the gas cost calculator and the percentage calculator for checking differences.
Frequently asked questions
How do I calculate tire diameter from the tire size?
Diameter = rim inches × 25.4 + 2 × width × aspect ratio ÷ 100 (in mm). For 205/55R16: 406.4 + 225.5 = 631.9 mm, or 24.88 inches.
Do bigger tires make the speedometer read low?
Yes. A larger circumference covers more distance per revolution. With a 2 % larger tire, 60 mph on the speedometer is really 61.2 mph.
What does 91V on a tire mean?
91 is the load index: 615 kg (1,356 lb) per tire. V is the speed rating: up to 240 km/h (149 mph). Both come from UN Regulation No. 30.
How much tire size difference is OK?
There is no single worldwide percentage. Stick to the sizes on your tire placard or approved by the manufacturer, and never go below the original load index and speed rating.
Why does a measured circumference differ from the calculation?
The calculator uses the nominal size of a new tire. Under load, with worn tread or low pressure, the effective rolling circumference is a little smaller.
Sources and legal basis
- UN R30: UN Regulation No. 30, Rev. 3 – tyre dimensions (para. 6.1.2.1: D = d + 2H), speed symbols (para. 2.29.1), load-capacity indices (Annex 4); UNECE PDF, archived copy (checked 27 Sep 2026)
- UN R39: UN Regulation No. 39, Rev. 2 – speedometer: indicated speed never below true speed, 0 ≤ V1 − V2 ≤ 0.1 V2 + 4 km/h (para. 5.4); UNECE PDF, archived copy (checked 27 Sep 2026)
As of:
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