Reference chart
Tire Size Chart: How to Read the Code
By &Pixels, the studio that builds MyCalculator.to. Load and speed figures are the standard industry tables. The placard in your door frame is the authority for your own car.
225/45R17 94V decoded, with the load and speed tables. The code looks arbitrary because its numbers are in three different units.
What does a tire size mean?
A tyre size is four measurements in one string, and the reason it reads as nonsense is that they are not in the same units. In 225/45R17, the 225 is millimetres of tread width, the 45 is a percentage rather than a measurement, and the 17 is inches of wheel diameter.
So the sidewall on that tyre is 45 percent of 225 mm, about 101 mm tall. Change the width and the same aspect ratio gives a different sidewall, which is why swapping one number in the code moves the overall diameter more than people expect.
Reading 225/45R17 94V
| Part | Here | What it means |
|---|---|---|
| Width | 225 | tread width in millimetres |
| Aspect ratio | 45 | sidewall height as a percentage of the width |
| Construction | R | radial, which is nearly every modern tyre |
| Rim diameter | 17 | wheel diameter in inches |
| Load index | 94 | a code for maximum load, 94 is 670 kg |
| Speed rating | V | a code for maximum speed, V is 149 mph |
The calculators behind this
Speed ratings
| Code | Max mph | Max km/h |
|---|---|---|
| Q | 99 | 160 |
| S | 112 | 180 |
| T | 118 | 190 |
| H | 130 | 210 |
| V | 149 | 240 |
| W | 168 | 270 |
| Y | 186 | 300 |
Load index
| Index | Per tyre, kg | Per tyre, lb |
|---|---|---|
| 85 | 515 | 1,135 |
| 88 | 560 | 1,235 |
| 91 | 615 | 1,356 |
| 94 | 670 | 1,477 |
| 97 | 730 | 1,609 |
| 100 | 800 | 1,764 |
| 105 | 925 | 2,039 |
How to read yours
- Read the width in millimetres. The first number. 225 means 225 mm across the tread, not inches and not the whole tyre.
- Treat the second number as a percentage. A 45 aspect ratio means the sidewall is 45 percent of 225 mm, about 101 mm tall.
- Read the diameter in inches. The number after the R. This one is imperial while the first two are metric.
- Keep the overall diameter within about 3 percent. Beyond that the speedometer reads wrong and on many cars the electronics notice.
- Match or exceed the load and speed ratings. These are on the placard in the door frame. Going below either is the one change never to make.
For your own car, the tire pressure calculator works from the placard figures, and the VIN decoder will tell you what the vehicle left the factory with, which is worth checking if the tyres on it now were fitted by somebody else.
The placard inside the driver’s door frame is the authority for your vehicle. These tables explain the code; they do not override what the manufacturer specified.
Formula & Methodology
Did you know? Speed rating letters are not in alphabetical order. H sits between U and V rather than after G, because it was introduced as a high-speed rating before the rest of the scale was extended around it and was never renamed.
Sources
- U.S. Tire Manufacturers Association, tyre sidewall marking references
- National Highway Traffic Safety Administration, tyre labelling and safety guidance
- ISO metric tyre designation standards for passenger vehicles