How to read a tire size
Take 265/70R17 — the most common light-truck size in America. Each part tells you something specific:
| Marking | Meaning | For 265/70R17 |
|---|---|---|
| 265 | Section width in millimetres, sidewall to sidewall | 265 mm (10.4 in) wide |
| 70 | Aspect ratio: sidewall height as a % of width | 70% of 265 = 185.5 mm (7.3 in) tall sidewall |
| R | Radial construction (nearly all modern tires; D = diagonal/bias) | Radial |
| 17 | Wheel (rim) diameter in inches | Fits a 17-inch wheel |
Overall diameter is then simple arithmetic: two sidewalls plus the wheel. 185.5 + 185.5 + (17 × 25.4) = 802.8 mm (31.6 in). Our tire size calculator does this for any two sizes.
Letters before the size: P, LT, ST
- P (P-metric) — passenger-car standard. Softer ride, lighter duty. Example: P265/70R17.
- LT (Light Truck) — heavier construction, higher load capacity, often with load range C/D/E. The right choice for towing and hauling.
- ST (Special Trailer) — trailer-only tires. Never put these on a drive axle.
- No letter (Euro metric) — same sizing system, built to European load standards. Common on modern trucks and SUVs.
Flotation sizes: 33x12.5R17
Truck and off-road tires sometimes use flotation sizing, which states the overall diameter directly: 33 = overall diameter in inches, 12.5 = section width in inches, R17 = radial, 17-inch wheel. A 33x12.5R17 is roughly equivalent to a 325/75R17 in metric terms — approximately, since flotation sizes are nominal.
Load index and speed rating
After the size you'll see something like 115T. The number is the load index (max weight per tire); the letter is the speed rating (max sustained speed).
| Load index | Max load per tire |
|---|---|
| 110 | 2,337 lb (1,060 kg) |
| 113 | 2,535 lb (1,150 kg) |
| 115 | 2,679 lb (1,215 kg) |
| 120 | 3,086 lb (1,400 kg) |
| 126 | 3,748 lb (1,700 kg) |
| Speed rating | Max speed |
|---|---|
| S | 112 mph (180 km/h) |
| T | 118 mph (190 km/h) |
| H | 130 mph (210 km/h) |
| V | 149 mph (240 km/h) |
Never downgrade load index. Your replacement tires must meet or exceed the load index on your door-jamb sticker — especially on trucks that tow or haul. This matters more than the diameter math.
Other sidewall markings worth knowing
- DOT code — the last four digits are the manufacture week/year (e.g., 2325 = week 23 of 2025). Tires age out around 6–10 years regardless of tread.
- 3PMSF (three-peak mountain snowflake) — meets a defined snow-traction test. Different from M+S, which is only a tread-pattern designation.
- UTQG — treadwear/traction/temperature grades. Useful for comparing within one brand; not comparable across brands.
- Max pressure — the tire's maximum, not your vehicle's recommended pressure. Use the door-jamb sticker for daily driving.
Advertisement