How to Read Tire Sidewall Markings: Size, Load Index, Speed Rating, DOT Age & 3PMSF
Complete engineering reference for ISO metric tire sizing (e.g., 225/45 R17 94W XL), DOT manufacture week/year, and winter 3PMSF homologation.
| Example Marking 225/45 R17 | 225 mm width · 45% aspect ratio · 17 in rim |
|---|---|
| Load Index 91 vs 94 XL | 91 = 615 kg (1,356 lb) · 94 = 670 kg (1,477 lb) |
| Speed Symbols H / V / W / Y | H=210 · V=240 · W=270 · Y=300 km/h |
| Maximum Tire Service Age | 6 years active use / 10 years max from DOT date |
1. Section Width, Aspect Ratio, and Rolling Diameter
In '225/45 R17 94W XL', 225 is the nominal section width in millimeters, 45 is the sidewall height as a percentage of width (225 × 0.45 = 101.25 mm), 'R' denotes radial ply construction, and 17 is the bead seat rim diameter in inches. When changing wheel sizes, keep overall rolling diameter within ±2.5% so ABS/ESC slip thresholds and speedometer calibration remain accurate.
2. DOTTIN Date Code and UNECE R117 3PMSF Symbol
The last four digits of the DOT code indicate manufacture week and year (e.g., '3825' = Week 38 of 2025). For winter and all-weather compliance in Europe (UNECE R117) and Canada, look for the Three-Peak Mountain Snowflake (3PMSF) symbol, which certifies controlled snow traction testing rather than geometric tread pattern alone (M+S).