How To Read Tire Size Codes | Numbers That Matter

Tire sidewall markings show width, profile, wheel diameter, load index, and speed rating in one compact code.

A tire code can look like a random string at first glance. Once you know the pattern, it reads like a label. You can tell how wide the tire is, how tall the sidewall stands, what wheel it fits, how much weight it can carry, and the speed class the maker assigned to it.

That matters when you’re checking what’s on your car now, shopping for replacements, or trying to spot a mismatch after a tire shop visit. A code such as P225/65R17 102H is not there for decoration. Each piece has a job, and missing one piece can lead to the wrong tire ending up on the wheel.

How To Read Tire Size Codes On A Sidewall Without Guesswork

Start at the largest string of letters and numbers on the sidewall. On many passenger vehicles, it will look close to this: P225/65R17 102H. Read it from left to right.

• P = tire type
• 225 = width in millimeters
• 65 = sidewall height as a percent of width
• R = radial build
• 17 = wheel diameter in inches
• 102 = load index
• H = speed rating

That one line gives you the tire’s physical fit and a chunk of its working limits. Read it once or twice, and the code stops feeling cryptic. The trick is to treat each break in the string as a separate label, not one long part number.

What Each Part Of The Code Tells You

The Prefix Letter

The first letter shows the tire class. P means passenger. LT means light truck. ST shows a trailer tire. Sometimes you’ll see no prefix at all. In many retail listings, that points to a Euro-metric passenger tire. The tire may still fit the wheel, but the load rules are not always identical, so check the placard before swapping types.

The Three-Digit Width

The next number is the tire width in millimeters, measured across the sidewall at its standard rim width. A 225 tire is about 225 mm wide. Wider tires can add grip and fill the wheel well more, but width alone tells you nothing about sidewall height or wheel fit. That’s why a 225/65R17 and a 225/45R17 share width but behave in different ways on the road.

The Aspect Ratio

The two-digit number after the slash is the aspect ratio. It shows sidewall height as a percent of tire width. So on a 225/65R17 tire, the sidewall height is 65 percent of 225 mm. Lower numbers mean a shorter sidewall. That usually brings a firmer feel and sharper steering. Higher numbers bring more cushion and more sidewall flex.

The Construction Letter And Wheel Diameter

R means radial, which is what you’ll see on nearly every modern passenger vehicle tire. After that comes wheel diameter in inches. A tire marked R17 fits a 17-inch wheel, not a 16 and not an 18. This is the non-negotiable fit number. If wheel diameter is wrong, the tire will not mount correctly.

The Load Index And Speed Symbol

The last numbers and letter show what the tire can carry and the speed class attached to it. The load index is not pounds written out in plain text. It’s a code number tied to a chart. The speed symbol is a letter, such as H or V. Those two marks matter when you replace tires. Match the vehicle placard or manual, and do not drop below the spec your car was built around.

Code Part	What It Means	What To Check
P / LT / ST	Vehicle class the tire was built for	Passenger, light truck, and trailer tires are not interchangeable by habit
225	Section width in millimeters	Width affects fit, stance, and clearance
/	Break between width and profile	Read the numbers on each side as separate data points
65	Aspect ratio, or sidewall height as a percent of width	Lower ratio = shorter sidewall; higher ratio = taller sidewall
R	Radial construction	Common on modern road tires
17	Wheel diameter in inches	Must match the wheel exactly
102	Load index code	Use the placard or manual so the replacement tire carries enough weight
H	Speed rating symbol	Choose a rating that meets the vehicle maker’s spec

Where People Get Mixed Up

The sidewall has more than one code, and that’s where confusion starts. The tire size is only one cluster. Nearby, you may also see extra load marks, winter symbols, UTQG grades, the DOT string, and the inflation pressure printed on the tire itself. Those marks do not all answer the same question.

The door-jamb placard stays your final fit check. The NHTSA tire ratings and awareness page points drivers to the vehicle label or owner’s manual for the size and pressure the car was set up to use. That matters more than copying a random tire size from a similar trim parked next door.

Another snag is the habit of reading a lower-profile tire as “bigger.” It often looks bigger because the wheel fills more of the total package. Yet outside diameter can stay close across different sizes. A 225/65R17 and a 235/55R18 can land near each other in overall height while using different wheels and sidewall shapes.

You’ll also see service marks that sit outside the basic size. XL means extra load. M+S points to mud and snow use. The mountain-snowflake symbol marks a tire that passed a winter traction test. Those marks matter when you compare two tires with the same size code but different job descriptions.

If the DOT string catches your eye, save it for a second pass. Federal Tire Identification Number rules say that code carries plant, size symbol, maker code, and the week-year date stamp.

Tire Code	Plain-English Readout	Typical Vehicle Match
P215/60R16 95H	Passenger tire, 215 mm wide, medium sidewall, 16-inch wheel, mid-range load and speed class	Mainstream sedan or small crossover
LT265/70R17 121/118S	Light-truck tire, wider tread, taller sidewall, 17-inch wheel, heavier load rating	Pickup or SUV used for hauling
235/45ZR18 98Y	Passenger tire with short sidewall, 18-inch wheel, high-speed service description	Sport sedan or performance hatch
T125/70D16 96M	Temporary spare, narrow tread, bias build, 16-inch wheel	Compact spare only

Using The Code When You Buy Replacement Tires

If you’re buying new tires, the safest move is to start with the factory placard and then compare it with the sidewall on the tire you have now. If those two match, you’re in good shape. If they do not, slow down and find out why. The current tire may be an approved alternate size, or it may just be wrong.

Use this short check list before you order:

• Match the wheel diameter exactly.
• Stay with the vehicle maker’s listed size unless you know an approved alternate.
• Meet the required load index and speed symbol.
• Check clearance if you plan to go wider.
• Replace tires in complete axle pairs at a minimum.
• For staggered setups, confirm front and rear sizes separately.

If you drive in snow country, one more mark is worth your time. A tire can carry the same size code as another tire and still behave in a different way once temperatures drop. That’s where the winter symbols and compound choice come into play, not the width-profile-diameter string alone.

A Few Sidewall Marks Near The Size Code

The DOT string is not the size code, but it is handy when checking age and recall details. On modern tires, the last four digits show the build week and year. A code ending in 2324 means the tire was made in the 23rd week of 2024.

UTQG grades sit nearby on many passenger tires sold in the United States. Those treadwear, traction, and temperature grades are useful shopping data, but they are not part of the size code. Treat them as a separate line of information. Same story with max inflation and max load printed on the sidewall. Those marks do not replace the vehicle placard pressure.

Once you know which cluster is the actual size code, the rest of the sidewall gets easier to sort. Read left to right, split the code into chunks, then match what you see against the placard on the car. After that, the sidewall stops looking like alphabet soup and starts reading like a plain label.