Decoding Tire Size in 60 Seconds
The string of numbers and letters on your tire sidewall — like 265/70R17 121S — tells you everything about what that tire is designed to do. Once you understand the code, you can confidently compare tires, verify fitment, and avoid costly mistakes.
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The Full Tire Code Breakdown
Note on the prefix: Some tires begin with a letter prefix — P (Passenger), LT (Light Truck), or ST (Special Trailer). No prefix means the tire meets Euro metric standards. LT tires have higher load ratings and are common on trucks and tow vehicles.
Section Width — The First Number (265)
Measured in millimeters
- ✓The width of the tire from sidewall to sidewall at its widest point
- ✓Common passenger widths range from 185mm to 295mm
- ✓Wider tires provide more grip but can reduce fuel economy and increase road noise
- ✓This is the tire's width — NOT the tread contact width, which is narrower
Example: A 265mm-wide tire is about 10.4 inches across. Going from 265 to 275 adds roughly 0.4 inches (10mm) of width per tire.
Aspect Ratio — The Second Number (70)
Measured in % of width
- ✓The sidewall height as a percentage of the tire's section width
- ✓Example: 265 × 0.70 = 185.5mm of sidewall height
- ✓Lower numbers (35–55) = low-profile, sportier look, stiffer ride
- ✓Higher numbers (65–80) = more sidewall, better cushioning, common on trucks/SUVs
Changing aspect ratio changes the overall tire diameter. Going too wide or too tall can cause rubbing and throw off your speedometer.
Rim Diameter — The Number After R (17)
Measured in inches
- ✓The diameter of the wheel the tire is designed to fit — measured in inches
- ✓Must match your wheel exactly — a 17-inch tire will NOT fit a 16 or 18-inch wheel
- ✓Common sizes: 15", 16", 17", 18", 19", 20", 22"
- ✓Larger diameter wheels typically require lower aspect ratio tires to keep the same overall diameter
This is the one number you cannot fudge. Always match rim diameter exactly.
Load Index & Speed Rating (121S)
Load Index (121)
A numeric code representing the maximum weight the tire can support when properly inflated. Always match or exceed your vehicle's required load index.
| Index | Max Load |
|---|---|
| 95 | 1,521 lbs |
| 100 | 1,764 lbs |
| 110 | 2,337 lbs |
| 121 | 3,197 lbs |
| 128 | 3,748 lbs |
Speed Rating (S)
A letter indicating the maximum sustained speed the tire is designed for. Never use a tire rated lower than your vehicle requires.
| Rating | Max Speed | Common Use |
|---|---|---|
| S | 112 mph | Trucks, SUVs, family cars |
| T | 118 mph | Family cars, minivans |
| H | 130 mph | Sport sedans, coupes |
| V | 149 mph | Sports cars |
| W/Y | 168/186 mph | High-performance cars |
Where to Find Your Required Tire Size
Driver's Door Jamb
Most common location. Open the driver's door and look for the sticker on the door frame. Lists the OEM recommended tire size and inflation pressure.
Owner's Manual
The specifications section lists the original equipment tire size plus acceptable alternatives for different trim levels.
Fuel Door / Glove Box
Some vehicles print tire specs on the inside of the fuel door or glove box lid, especially trucks with dual load ratings.
Know Your Size? Find Matching Tires
Browse by size or enter your vehicle to see every matching tire with live prices.