📋 The Anatomy of a Tire Size
A tire size like P275/65R18 116T packs six pieces of data into one code.
| P | Service type. P = Passenger. LT = Light Truck. ST = Trailer. Blank = Euro-metric. |
| 275 | Section width in millimeters. |
| 65 | Aspect ratio. Sidewall height = 65% of 275mm = 179mm. |
| R | Construction. R = Radial (almost everything). D = Bias-ply. B = Bias-belted. |
| 18 | Wheel diameter in inches. |
| 116 | Load index. 116 = 2756 lb per tire. |
| T | Speed rating. T = up to 118 mph. |
📊 Load Index Table
| Index | Lb / Tire |
|---|---|
| 85 | 1135 |
| 91 | 1356 |
| 95 | 1521 |
| 100 | 1764 |
| 110 | 2337 |
| 116 | 2756 |
| 121 | 3197 |
🏎️ Speed Rating Table
| Code | MPH | Typical Use |
|---|---|---|
| Q | 99 | winter / off-road LT |
| S | 112 | SUV / pickup |
| T | 118 | pickup / family sedan |
| H | 130 | sport sedan / CUV |
| V | 149 | performance sedan |
| W | 168 | sports car |
| Y | 186 | supercar |
🔍 Other Sidewall Marks
DOT code: The last 4 digits = week and year of manufacture. "2823" = 28th week of 2023.
Treadwear / Traction / Temperature: UTQG rating. Higher treadwear = longer wear (relative). Traction AA / A / B / C. Temperature A / B / C.
M+S, 3PMSF: Mud and Snow rating, Three-Peak Mountain Snowflake (true severe-snow rated).
❓ Frequently Asked Questions
What does the R in a tire size mean?
R means radial construction. The plies run perpendicular to the direction of travel. Virtually all modern street tires are radial.
What is the difference between P and LT tires?
P (passenger) tires are tuned for ride comfort and lower load capacity. LT (light truck) tires have stiffer sidewalls, higher load capacity, and run higher pressures, suited for trucks, vans, and heavy SUVs.
What is aspect ratio on a tire?
Aspect ratio is the sidewall height as a percentage of section width. Lower = shorter sidewall = sportier handling but harsher ride.
Why do truck tires have higher load indexes?
Trucks carry more weight per tire. A load index too low for the vehicle's GVWR can cause sidewall failure under load.
How do I find my tire size?
Check the sidewall, the door-jamb tire-and-loading sticker, the owner's manual, or the fuel-filler door label.
Can I mix tire sizes on my car?
No, you should run the same size on all four (or matched front/rear for staggered cars). Mixing causes uneven wear, ABS issues, and AWD drivetrain damage on AWD vehicles.