Those letters and numbers on your tire are not random. Tire size codes describe the width, shape, construction and capability of the tire, and understanding them makes it easier to buy the right replacement and maintain what you have. This guide from Ran When Parked breaks down the most common markings in plain language.
The Main Size Code
A typical size code looks like a string of numbers and letters with a slash and a dash. Each part has a meaning. The first letters often show the tire type, such as one for passenger vehicles or one for light trucks. Then come the key measurements.
- Width: the first number is the tire’s section width in millimeters.
- Aspect ratio: the number after the slash is the sidewall height as a percentage of the width. A lower number usually means a shorter sidewall.
- Construction: a letter that shows how the tire is built, with radial being the most common.
- Wheel diameter: the number after the construction letter, in inches, which must match your wheel.
Load Index and Speed Rating
After the size you will often see a number and a letter, such as a two or three digit figure followed by a letter. The number is the load index, which shows how much weight the tire can carry. The letter is the speed rating, which indicates the maximum speed the tire is designed to sustain under test conditions.
These ratings are not decoration. Replacement tires should generally meet or exceed the ratings recommended by the vehicle maker. Check the placard and the owner’s manual, and ask a professional if you are considering a different size.
Other Useful Markings
Elsewhere on the sidewall you may find several other codes that are worth knowing.
- Treadwear, traction and temperature grades: comparative ratings assigned under standardized testing.
- Maximum inflation pressure: the most air the tire can hold, not the recommended pressure for your car.
- M+S or snow symbols: markings related to mud, snow or winter capability.
- DOT code: a series of characters that identifies the manufacturer and, in its final digits, when the tire was made.
The date portion of the DOT code is helpful when buying tires or checking a spare. Tires age even if they are barely used, so it is wise to know how old yours are.
Why Matching Matters
Using the wrong size or rating can affect handling, braking, speedometer accuracy and clearance. Mixing different tires on the same axle can also create uneven behavior. When replacing tires, follow the vehicle manufacturer’s recommendation, and consider having a tire shop confirm that your choice is suitable.
Remember that tire pressure should come from the vehicle placard, not the number molded on the tire. People often confuse the two.
A Quick Worked Example
Imagine a tire with a size code that begins with a three digit number, followed by a slash, a two digit number, a letter and a two digit number. The first number is the width in millimeters. The two digit number after the slash is the aspect ratio, so the sidewall height is that percentage of the width. The letter shows the construction, and the last number is the wheel diameter in inches.
Right after that you may see the load index and speed rating. Together, these tell you the tire’s size, shape and capability at a glance. If any one of these elements differs from what your vehicle calls for, ask a professional before purchasing.
Take a photo of the sidewall markings and the door placard with your phone. Having them handy makes it easy to compare options when you are shopping online or talking to a tire shop.
If the markings are hard to read, clean the sidewall with a cloth and look at an angle in good light.
Tire Code Checklist
- Find the size code on the sidewall.
- Compare it with the door-jamb placard and owner’s manual.
- Note the load index and speed rating.
- Check the DOT date code for age.
- Confirm all tires on the vehicle are appropriate and matched.
- Use the placard pressure, not the maximum inflation number.
- Ask a tire professional if you want to change sizes.
Frequently Asked Questions
Can I use a different tire size than the original?
Sometimes, but it can affect handling, clearance and speedometer accuracy. Ask a professional and consult your manual first.
What does the tire age code tell me?
The last digits of the DOT code show the week and year of manufacture, which helps you judge how old a tire is.
Is a higher speed rating always better?
Not necessarily. Choose ratings that meet the manufacturer’s recommendation, and consider how the tire fits your driving needs.
