You need new tires, and the shop asks for your tire size. On the sidewall, you find something like “P225/65R17 102H” and a long code starting with “DOT.” It looks like a serial number, but each part tells you something important about the tire.
Key Takeaways
- The size code describes width, sidewall height ratio, construction and wheel diameter.
- The load index and speed rating show how much weight a tire can carry and its maximum rated speed; replacements should meet or exceed the original specification.
- The last four digits of the DOT code show the week and year of manufacture.
- Your vehicle’s correct tire size and pressure are listed on a placard, usually on the driver’s door jamb.
The size code
Take the example P225/65R17 102H.
P — Tire type. “P” means passenger car tire in the P-metric system. “LT” means light truck. Many tires use the European metric format with no letter.
225 — Section width in millimetres, measured from sidewall to sidewall.
65 — Aspect ratio: the sidewall height as a percentage of the width. Here, the sidewall height is 65% of 225 mm. Lower numbers mean shorter sidewalls, common on performance cars.
R — Construction. “R” means radial, which is standard for modern tires.
17 — Wheel diameter in inches. The tire fits a 17-inch wheel.
Load index
The 102 in the example is the load index. It corresponds to the maximum load each tire can carry when properly inflated. A load index of 102 corresponds to 850 kilograms (about 1,874 pounds) per tire.
Replacement tires should have a load index equal to or greater than the original equipment specification. Using a lower load index can be unsafe, especially when the vehicle is fully loaded or towing.
Some tires are marked “XL” (extra load) or “RF” (reinforced), meaning they can carry more load at higher pressures.
Speed rating
The H is the speed rating, indicating the maximum speed the tire can sustain under specified conditions. Common ratings include:
- S: 112 mph (180 km/h)
- T: 118 mph (190 km/h)
- H: 130 mph (210 km/h)
- V: 149 mph (240 km/h)
- W: 168 mph (270 km/h)
- Y: 186 mph (300 km/h)
Speed ratings also relate to handling characteristics. Replacement tires should generally match or exceed the original rating.
The DOT code and manufacturing date
The DOT code shows the tire meets US Department of Transportation standards and identifies the manufacturer and plant. The last four digits show when the tire was made:
- “3524” means the 35th week of 2024.
Tires age even when not used, as rubber compounds degrade over time. Many vehicle and tire manufacturers recommend inspecting tires regularly after about five years and replacing them after six to ten years regardless of tread depth. Check your vehicle and tire manufacturer’s guidance.
When buying tires, check the date code. A “new” tire that has been in storage for years may have less useful life.
Other markings
Treadwear, traction and temperature (UTQG). Many passenger tires show a treadwear number (such as 500), a traction grade (AA, A, B or C) and a temperature grade (A, B or C). Treadwear numbers compare tires within the same brand but are not precise lifetimes.
M+S. Mud and snow — indicates some all-season capability.
Three-peak mountain snowflake. Indicates the tire meets a performance standard for snow traction. Dedicated winter tires carry this symbol, and some all-weather tires do too.
Maximum pressure. The maximum inflation pressure the tire can hold. This is not the recommended pressure for your car.
Recommended tire pressure
Your vehicle’s recommended tire pressure is on the tire placard, usually on the driver’s door jamb, and in the owner’s manual. Check pressure monthly with a gauge when tires are cold. Under-inflation increases wear and heat buildup and reduces fuel economy.
Choosing replacements
- Match the size on the placard unless an approved alternative is listed.
- Match or exceed load index and speed rating.
- Replace in pairs or sets, especially on all-wheel-drive vehicles, where uneven tread depth can stress the drivetrain.
- Consider your climate — all-season, all-weather or winter tires.
Frequently Asked Questions
Can I use a different tire size? Only sizes approved by the vehicle manufacturer or a qualified specialist. Different sizes can affect speedometer accuracy, handling and safety systems.
Is a higher speed rating always better? Higher ratings can offer sharper handling but may wear faster or ride more firmly.
What does “XL” mean? Extra load — the tire can carry more weight at higher inflation pressures.
Where is the DOT date code? It may be on only one side of the tire. Check both sides.
The bottom line
The sidewall tells you a tire’s size, load capacity, speed capability and age. Use it to choose replacements that meet your vehicle’s specifications, and check the date code to avoid old stock.