How to read a tire size

The size, piece by piece

Take P225/65R17 98H, the size in the diagram above, and read it left to right. Passenger and light truck tires written this way all follow the same order, so once you can read one, you can read yours.

P: the vehicle type

P means a passenger car tire. LT means light truck: built to carry heavier loads, usually at higher pressures. ST means special trailer, and it belongs on a trailer only. A size with no letter in front is usually a metric passenger tire, read the same way.

225: the width

225 is the width of the tire in millimeters, measured from the widest point of one sidewall to the widest point of the other. That is a little under 9 inches.

65: the aspect ratio

65 is not a measurement. It is the height of the sidewall as a percentage of the width. Here is the math for this size:

• 225 mm x 0.65 = about 146 mm of sidewall, from the wheel to the tread.
• 146 mm is about 5.8 inches.
• Two sidewalls plus the 17 inch wheel give an overall height of about 28.5 inches.

A lower aspect ratio means a shorter sidewall. A 225/45 is the same width as a 225/65, but its sidewall is only about 101 mm tall.

R: the construction

R means radial: the cords inside run straight across the tire from one edge to the other. Almost every modern car and light truck tire is radial.

17: the wheel diameter

17 is the diameter of the wheel the tire fits, in inches. It has to match exactly. A 17 inch tire does not go on a 16 or an 18 inch wheel.

Truck sizes in inches

Some light truck tires use a different style that gives the overall height and width in inches, such as 31x10.50R15. The R and the last number still mean radial and wheel diameter.

Load index and speed rating

The 98H after the size is called the service description. 98 is the load index: a number that stands for the most weight one tire can carry at its rated pressure, looked up on a standard industry chart. H is the speed rating: a letter that stands for the top speed the tire is built to hold while carrying that load.

The rule for a replacement is short. Match or beat what the car calls for on both. Do not go below the load index or the speed rating listed on your door placard or in the owner's manual, even when a lower one is easier to find. On a pickup or van that hauls weight, the load number matters more than anything else on the sidewall.

A speed rating describes what the tire can handle. It is not a suggestion for how fast to drive.

Other markings on the sidewall

M+S and the mountain snowflake

M+S means mud and snow. It describes the tread pattern, and many all-season tires carry it. The three-peak mountain snowflake symbol is the stricter of the two: it means the tire met a snow traction test. Both are about winter grip, so they matter most if you drive somewhere with real winter.

UTQG ratings

Many passenger tires carry three UTQG ratings from a federal program. Treadwear is a number compared with a test tire, so a higher number points to longer wear in that test. Traction is a letter (AA, A, B or C) for grip on wet pavement. Temperature (A, B or C) is about how well the tire resists heat. The ratings are most useful for comparing tires from the same maker.

Maximum pressure

Near the bead you will see a maximum pressure, usually in psi and kPa. That is the most the tire can safely hold, a limit for the tire. It is not the pressure your car should run. Your car's number is on the driver's door placard.

How to check tire pressure the right way

Arrows and outside markings

A directional tire has an arrow showing which way it should roll, so it has to be mounted to turn that way on its side of the car. An asymmetric tire says outside on one sidewall, and that side has to face out. We follow these markings when we mount tires. They also limit which positions a tire can move to in a rotation.

If you cannot read the tire

Sidewall letters wear down and get scuffed on curbs. If yours are hard to read, the size is written in other places too.

• The placard on the driver's door jamb. It lists the size, load and pressure the car came with.
• Your owner's manual, usually in the tire or specifications section.
• Inside the fuel door, on some cars.
• With us. Give us the year, make and model and we can look up the size the car came with.

The sidewall tells you what is on the car now. The placard tells you what the maker fitted. If a past owner changed sizes, the two will not match, so check both before you buy.

Changing to a different size

Tires on the same axle should be the same size. Some cars come from the factory with wider tires on the rear than on the front. The placard shows both, and those tires stay on their own axle.

Most size changes happen with new wheels. The usual approach is to go up in wheel diameter and down in aspect ratio, so the overall height of the tire stays close to the original. That keeps the speedometer reading close and the tire clear of the fender and suspension. A size that looks close on paper is not always close enough, especially if the load index drops.

Ask us before you change. We check what fits the car before anything goes on it.

Wheels: what has to match for a wheel to fit

Once you have the size

Write down the whole size, including the load index and speed rating. With that we can show you used tires in the size, quote new ones from the major brands, and mount and balance them while you wait.

Used tires for cars, SUVs and light trucks

New tires by quote