Tire Size Numbers Explained: Reading the Code on Your Tires
Published Apr 14, 2026 · 5 min read
Every tire has a code stamped on its sidewall like P225/65R17 91H. It's not random — each part tells you exactly what the tire fits and how it performs.
Breaking Down P225/65R17 91H
| Code | Meaning | Value |
|---|---|---|
| P | Type | Passenger vehicle (LT = Light Truck) |
| 225 | Width | 225 mm tread width |
| 65 | Aspect ratio | Sidewall height = 65% of width (146 mm) |
| R | Construction | Radial (virtually all modern tires) |
| 17 | Rim diameter | Fits 17-inch wheels |
| 91 | Load index | Max 1,356 lbs per tire |
| H | Speed rating | Max 130 mph sustained |
What Each Number Affects
- Width (225): Wider = more grip, more road noise, more fuel consumption. Narrower = better fuel economy, better in snow.
- Aspect ratio (65): Lower = sportier handling, harsher ride. Higher = more comfortable, more sidewall flex.
- Rim diameter (17): Must match your wheel exactly. No flexibility here.
Speed Ratings
| Rating | Max Speed | Common Use |
|---|---|---|
| S | 112 mph | Family sedans |
| T | 118 mph | Family sedans, minivans |
| H | 130 mph | Sport sedans |
| V | 149 mph | Sports cars |
| W | 168 mph | Performance vehicles |
Can You Change Tire Size?
Within limits, yes — but keep the overall diameter within 3% of the original to prevent speedometer errors and clearance issues. The "plus zero" approach: go one inch up on rim size, reduce aspect ratio to keep the same overall diameter.
- P225/65R17 (overall diameter: 28.5")
- P235/55R18 (overall diameter: 28.2") ✅ within 3%
- P245/45R18 (overall diameter: 26.7") ❌ too small
Try it: Use our Tire Size Calculator to compare tire sizes and see how changes affect diameter and speedometer accuracy.