How to read a tire size
In P225/65R17 102H, P means a passenger tire, 225 is the width in millimeters, 65 is the sidewall height as a percentage of that width, R means radial and 17 is the wheel size in inches. 102H is the load index and speed symbol.[1 · Oct 29, 2026]
Each part of the size, left to right
| Part | Example | What it means |
|---|---|---|
| Tire type | P | P = passenger (P-metric). LT = light truck. ST = special trailer. T = temporary spare. No letter means a Euro-metric size, which may be rated for a different load than the P-metric size with the same numbers.[1 · Oct 29, 2026] |
| Section width | 225 | The approximate width from sidewall to sidewall, in millimeters.[1 · Oct 29, 2026] |
| Aspect ratio | 65 | Sidewall height as a percentage of the width. A lower number means a shorter sidewall.[1 · Oct 29, 2026] |
| Construction | R | R = radial. B, X or a dash means bias-ply; vehicle makers also use D for diagonal (bias) construction.[1 · Oct 29, 2026][2 · Oct 29, 2026] |
| Rim diameter | 17 | The wheel diameter the tire fits, in inches.[1 · Oct 29, 2026] |
| Load index | 102 | A code for the maximum load one tire is rated to carry. 102 stands for 1,874 lb (850 kg). See the load index chart.[1 · Oct 29, 2026][3 · Oct 29, 2026] |
| Speed symbol | H | The tire's speed category. H stands for 130 mph (210 km/h). See the speed rating chart.[1 · Oct 29, 2026][4 · Oct 29, 2026] |
Turning the code into inches
The size code is enough to work out the tire's nominal dimensions. For P225/65R17:
- Sidewall height: 225 × 0.65 = 146.25 mm, or 5.76 in.
- Overall diameter: 17 in wheel + two sidewalls = 28.52 in (724 mm).
- Section width: 225 ÷ 25.4 = 8.86 in.
- Revolutions per mile: about 707.
Tire Rack shows the same formula with 225/45R17, which works out to 24.98 in tall.[5 · Oct 29, 2026] Overall diameter is measured on an inflated tire that isn't carrying a load, so the height under a car is a little less.[6 · Oct 29, 2026]
The math (our own arithmetic): sidewall height = width × aspect ratio ÷ 100; overall diameter = rim × 25.4 + 2 × sidewall (mm), ÷ 25.4 for inches; revs per mile = 63,360 ÷ (π × diameter in inches); true speed = indicated speed × new diameter ÷ original diameter. These are nominal figures from the size code. Tire Rack notes that real tires differ from the formula, so use the tire maker's published spec when clearance is tight.[5 · Oct 29, 2026]
Other size formats you'll see
- LT sizes with two load numbers, such as LT235/75R15 104/101S: the first number is the load for a single tire and the second is for a tire in a dual-wheel pair (1,984 lb and 1,819 lb here).[3 · Oct 29, 2026]
- Flotation sizes, such as 31x10.5 R15 LT: about 31 in overall diameter, 10.5 in section width, for a 15 in wheel.[2 · Oct 29, 2026]
- Temporary spares, such as T145/80D18 103M: T marks a temporary-use spare, and D marks diagonal (bias) construction.[2 · Oct 29, 2026]
- ZR in the size, such as 245/40ZR18: the Z speed category, meaning over 149 mph (240 km/h).[4 · Oct 29, 2026]
- Commercial "C" sizes, such as 225/75R16C: a Euro-metric van and light truck size with its own load ranges.[7 · Oct 29, 2026]
Where to find the size your vehicle needs
Look on the Tire and Loading Information label, usually on the edge of the driver's door or the door post, or in the owner's manual. NHTSA says to buy the same size as the original tires or another size the vehicle maker recommends, and to ask a dealer if you're not sure.[8 · Oct 29, 2026] The same label gives the correct cold tire pressure, which isn't the maximum pressure molded on the tire.[8 · Oct 29, 2026]
The rest of the sidewall
- DOT code: the last four digits are the week and year it was made, so 2910 means week 29 of 2010.[1 · Oct 29, 2026] See tire date codes and UTQG.
- UTQG: treadwear, traction and temperature grades on passenger tires.[1 · Oct 29, 2026]
- Mountain and snowflake symbol: the tire passed a snow traction test.[1 · Oct 29, 2026]
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Sources: 1. Tire Industry Association, Reading a tire sidewall. 2. Ram, 2022 1500 tire and wheel supplement (PDF). 3. Tire Rack, What is load index?. 4. Tire Rack, How do I check a tire's speed rating?. 5. Tire Rack, How do I calculate tire dimensions?. 6. Tire Rack, How do I measure a tire's overall diameter?. 7. Tire Rack, What are load ranges and ply ratings?. 8. NHTSA, Tires. Checked Oct 29, 2026. Size dimensions, revolutions per mile and speedometer figures are our own arithmetic from the size code. Trademarks belong to their owners; figures are restated in our own words and tables, not reproduced.