Calculator
Tire size calculator
Read the whole sidewall, not just three numbers. Type a size as printed and get its dimensions, load capacity and speed rating, then compare it with a second size.
Dimensions
Comparison
| Measure | Size A | Size B | Difference |
|---|
Speedometer error
If the speedometer was calibrated for size A and the car is on size B:
| Speedometer reads | True speed |
|---|
This tool computes geometry. It cannot see your brakes, fenders or suspension. Confirm any fitment with a tire shop before you buy.
Load index table
The load index is the two or three digits after the size. It is the maximum weight one tire carries at its rated pressure.
| Index | kg | lb | Index | kg | lb |
|---|---|---|---|---|---|
| 60 | 250 | 551 | 96 | 710 | 1565 |
| 61 | 257 | 567 | 97 | 730 | 1609 |
| 62 | 265 | 584 | 98 | 750 | 1653 |
| 63 | 272 | 600 | 99 | 775 | 1709 |
| 64 | 280 | 617 | 100 | 800 | 1764 |
| 65 | 290 | 639 | 101 | 825 | 1819 |
| 66 | 300 | 661 | 102 | 850 | 1874 |
| 67 | 307 | 677 | 103 | 875 | 1929 |
| 68 | 315 | 694 | 104 | 900 | 1984 |
| 69 | 325 | 717 | 105 | 925 | 2039 |
| 70 | 335 | 739 | 106 | 950 | 2094 |
| 71 | 345 | 761 | 107 | 975 | 2150 |
| 72 | 355 | 783 | 108 | 1000 | 2205 |
| 73 | 365 | 805 | 109 | 1030 | 2271 |
| 74 | 375 | 827 | 110 | 1060 | 2337 |
| 75 | 387 | 853 | 111 | 1090 | 2403 |
| 76 | 400 | 882 | 112 | 1120 | 2469 |
| 77 | 412 | 908 | 113 | 1150 | 2535 |
| 78 | 425 | 937 | 114 | 1180 | 2601 |
| 79 | 437 | 963 | 115 | 1215 | 2679 |
| 80 | 450 | 992 | 116 | 1250 | 2756 |
| 81 | 462 | 1019 | 117 | 1285 | 2833 |
| 82 | 475 | 1047 | 118 | 1320 | 2910 |
| 83 | 487 | 1074 | 119 | 1360 | 2998 |
| 84 | 500 | 1102 | 120 | 1400 | 3086 |
| 85 | 515 | 1135 | 121 | 1450 | 3197 |
| 86 | 530 | 1168 | 122 | 1500 | 3307 |
| 87 | 545 | 1202 | 123 | 1550 | 3417 |
| 88 | 560 | 1235 | 124 | 1600 | 3527 |
| 89 | 580 | 1279 | 125 | 1650 | 3638 |
| 90 | 600 | 1323 | 126 | 1700 | 3748 |
| 91 | 615 | 1356 | 127 | 1750 | 3858 |
| 92 | 630 | 1389 | 128 | 1800 | 3968 |
| 93 | 650 | 1433 | 129 | 1850 | 4079 |
| 94 | 670 | 1477 | 130 | 1900 | 4189 |
| 95 | 690 | 1521 |
Speed rating table
The letter after the load index. It is the maximum speed the tire is rated to sustain, not a recommendation.
| Symbol | mph | km/h |
|---|---|---|
| L | 75 | 120 |
| M | 81 | 130 |
| N | 87 | 140 |
| P | 93 | 150 |
| Q | 99 | 160 |
| R | 106 | 170 |
| S | 112 | 180 |
| T | 118 | 190 |
| U | 124 | 200 |
| H | 130 | 210 |
| V | 149 | 240 |
| W | 168 | 270 |
| Y | 186 | 300 |
The math, shown
Sidewall = width Γ aspect Γ· 100. Diameter = wheel Γ 25.4 + 2 Γ sidewall. Circumference = Ο Γ diameter. Revolutions per mile = 1,609,344 Γ· circumference in mm. True speed = indicated Γ new diameter Γ· old diameter.
Questions people ask
How do I read a tire size like 225/55R18 98H?
225 is the section width in millimetres. 55 is the aspect ratio: the sidewall is 55% of the width. R means radial. 18 is the wheel diameter in inches. 98 is the load index, 750 kg per tire from the table on this page. H is the speed rating, 130 mph. A P in front means a passenger tire built to U.S. standards; LT means light truck.
What is the 3% rule?
A replacement tire should keep the overall diameter within 3% of the factory size. The speedometer, odometer, ABS and stability control all convert wheel rotation to speed using the factory diameter, and gearing and clearance were designed around it. Under 2% is the conservative target.
Does a bigger tire make my speedometer read low?
Yes. A larger diameter covers more ground per revolution, so at an indicated 60 mph you are travelling faster than 60. The table on this page shows the true speed for each indicated speed. The odometer under-counts by the same fraction.