Every tyre has a series of numbers and letters on its sidewall that might look like a secret code: P225/45R17 91W. But once you understand the system, it becomes easy to read and ensures you always get the right tyres for your vehicle.
Breaking Down the Code
Vehicle Type
P = Passenger car tyre (most common)
LT = Light Truck
ST = Special Trailer
T = Temporary (spare tyre)
No letter = Euro-metric (same as P for most purposes)
Section Width (mm)
The tyre's width from sidewall to sidewall, measured in millimeters. Common widths range from 155mm (compact cars) to 335mm (performance vehicles).
Wider tyres = more grip but higher fuel consumption and cost
Aspect Ratio (%)
The sidewall height as a percentage of the width. A 225/45 tyre has a sidewall height of 225 × 0.45 = 101.25mm.
Lower ratio (30-45) = sportier handling, harsher ride
Higher ratio (55-75) = more comfortable, better protection from potholes
Construction Type
R = Radial construction (99%+ of modern tyres)
D = Diagonal/Bias construction (vintage vehicles, some trailers)
B = Belted (rare)
Radial tyres have layers running perpendicular to the direction of travel, providing better flexibility, fuel efficiency, and tread life.
Rim Diameter (inches)
The diameter of the wheel the tyre fits, measured in inches. This must match your wheel exactly. Common sizes: 14" to 22".
Larger rims often mean lower-profile tyres for the same overall diameter
Load Index
The maximum weight each tyre can safely carry. 91 = 615 kg (1356 lbs) per tyre.
Never use tyres with a lower load index than your vehicle requires. Check your door jamb sticker for the minimum.
Speed Rating
The maximum safe speed for the tyre. W = 270 km/h (168 mph).
Common ratings: S (180), T (190), H (210), V (240), W (270), Y (300)
Load Index Reference Table
| Index | Load (kg) | Load (lbs) | Index | Load (kg) | Load (lbs) |
|---|---|---|---|---|---|
| 75 | 387 | 853 | 95 | 690 | 1521 |
| 80 | 450 | 992 | 100 | 800 | 1764 |
| 85 | 515 | 1135 | 105 | 925 | 2039 |
| 90 | 600 | 1323 | 110 | 1060 | 2337 |
Full load index chart available at Tire Rack
Speed Rating Reference Table
| Rating | Max Speed (km/h) | Max Speed (mph) | Typical Use |
|---|---|---|---|
| S | 180 | 112 | Family sedans, minivans |
| T | 190 | 118 | Family sedans, minivans |
| H | 210 | 130 | Sport sedans, coupes |
| V | 240 | 149 | Sports cars, performance sedans |
| W | 270 | 168 | Exotic sports cars |
| Y | 300 | 186 | Supercars |
| (Y) | 300+ | 186+ | Hypercars |
Where to Find Your Tyre Size
1. Door Jamb Sticker
Open the driver's door and look for a sticker on the door frame or pillar. This shows the manufacturer's recommended tyre size and pressure.
2. Owner's Manual
The tyre specifications section lists acceptable sizes, including any optional sizes approved by the manufacturer.
3. Current Tyre Sidewall
If the current tyres are original or properly fitted, the size is molded into the sidewall.
When shopping for tyres online, use tools like Tire Rack's fitment guide to verify compatibility with your specific vehicle year, make, and model.
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