How to Read a Tire Size: 225/45R17 Decoded
A tire size like 225/45R17 91W crams three unit systems and five facts into eleven characters. Once you can read it fluently, you can compare any two tires, predict speedometer error, and spot a bad fitment suggestion from across the room. Here's the full decode.
The main code: 225/45R17
| Part | Name | Meaning |
|---|---|---|
| 225 | Section width | Widest sidewall-to-sidewall measurement in millimeters, on the design rim |
| 45 | Aspect ratio | Sidewall height as a percentage of the width — here 45% of 225mm = 101mm |
| R | Construction | Radial. Practically everything modern is; D (diagonal/bias) survives on trailers and classics |
| 17 | Rim diameter | Wheel diameter it mounts on, in inches |
Yes: width in millimeters, sidewall as a ratio, diameter in inches. The mixed units are a historical accident everyone is stuck with. The one number the code doesn't state directly — and the one fitment cares most about — is overall diameter:
For 225/45R17: 431.8 + 2 × 101.25 = 634mm (25.0"). Any replacement size within about 1–2% of that keeps your speedometer honest and your ABS/AWD electronics unbothered.
The service description: 91W
- 91 — load index. A lookup code for maximum load per tire: 91 = 615kg. Never go below your OEM load index; heavier wheels and lower sidewalls don't change the car's weight.
- W — speed rating. Sustained speed capability: H = 210 km/h, V = 240, W = 270, Y = 300, (Y) = 300+. Insurance and inspection in some countries require matching or exceeding OEM.
Other markings worth recognizing: XL (extra load — higher pressure for the same load index), RF/ROF/RFT (run-flat), M+S and the mountain-snowflake (winter capability), and a four-digit DOT date code like 2325 — week 23 of 2025. Tires older than ~6 years are due regardless of tread.
Plus-sizing: changing rim diameter without changing rollout
Going from 16s to 17s or 18s works by trading sidewall for rim — one inch more rim, roughly 10 points less aspect ratio, often 10mm more width. Each step keeps overall diameter nearly constant:
| Size | Overall diameter | vs base |
|---|---|---|
| 205/55R16 | 632mm | — |
| 225/45R17 | 634mm | +0.3% ✓ |
| 235/40R18 | 645mm | +2.1% ⚠ borderline |
| 225/40R18 | 637mm | +0.8% ✓ |
The pattern generalizes: pick the width you want, then choose the aspect ratio that lands the diameter closest to stock. That's exactly the computation behind the tire suggestions on our car pages, and the calculator shows the resulting speedometer error for any combination you type.
Width vs rim width
Every tire width has an approved rim-width range (on the sidewall's fine print or the manufacturer's spec sheet). A 225 typically approves 7–8.5" rims: below the range the tire crowns and wears its center; above it the sidewalls pull taut — stretch, with everything that implies. Matching tire width to rim width is half of fitment; the other half is where that package sits, which is offset's job.