The sidewall code tells you a tire’s width, profile, build, wheel size, load rating, speed rating, and build date.
If the string of letters and numbers on your tire feels like gibberish, you’re not alone. Most drivers see something like P225/65R17 102H and move on. Once you know the order, the code gets plain fast: each piece tells you what the tire fits, how tall the sidewall is, how much weight it can carry, and how fast it is rated to run.
That matters when you’re buying replacements, checking if a used set fits your car, or trying to make sense of what a tire shop is offering. Read the sidewall in the right order, and you can spot a mismatch before it costs you money, ride quality, or grip.
What Are The Numbers On A Tire? Read The Sidewall In Order
Start with the main size code. On a passenger tire, it often looks like this: P225/65R17 102H. Each chunk has a job. You don’t need to memorize the whole alphabet. You just need to know what each part points to.
The First Letter And The First Three Digits
The opening letter tells you the tire type. P means passenger. You may also see LT for light truck or no letter at all on some metric-sized tires. Right after that comes the section width in millimeters. In this case, 225 means the tire is about 225 mm wide from sidewall to sidewall.
A wider tire can change steering feel, fuel use, and clearance.
The Two Digits After The Slash
The next number is the aspect ratio. Here, 65 means the sidewall height is 65% of the tire’s width. Lower numbers mean a shorter sidewall. That usually brings a firmer feel and a sharper response. Higher numbers bring more cushion and a taller sidewall.
The Letter Before The Wheel Size
The R stands for radial construction. That’s what you’ll see on almost every modern passenger vehicle tire. After that comes 17, which is the wheel diameter in inches. If your wheel is 17 inches, that number must match.
One catch: the width is in millimeters, the wheel size is in inches, and the aspect ratio is a percentage.
Reading The Numbers On A Tire On A Real Example
Say your sidewall reads 235/55R18 100V. Read it left to right:
- 235 = tire width in millimeters
- 55 = sidewall height as 55% of the width
- R = radial construction
- 18 = wheel diameter in inches
- 100 = load index
- V = speed rating
Read it that way once, and the code gets a lot easier to follow.
The first four pieces handle basic fit. The last two tell you weight and speed class.
Numbers Past The Size Code Still Matter
After the wheel size, you’ll often see a two- or three-digit load index and a letter speed rating. Michelin’s tire markings explainer lays out the same reading order used across modern passenger tires. The load index is a coded value tied to how much weight one tire can carry when inflated the right way. The speed letter is the tire’s tested speed category at that load.
A tire can match your old width and wheel size and still be wrong if the load index or speed rating drops below spec.
| Sidewall Mark | What It Means | Why You Should Check It |
|---|---|---|
| P / LT / None | Tire type: passenger, light truck, or euro-metric style | Sets the tire’s intended vehicle class and load style |
| 225 | Section width in millimeters | Changes fit, steering feel, and clearance |
| 65 | Aspect ratio, or sidewall height as a share of width | Changes ride feel and overall tire height |
| R | Radial construction | Standard build on modern road tires |
| 17 | Wheel diameter in inches | Must match the wheel exactly |
| 102 | Load index code | Tells how much weight one tire can carry |
| H | Speed rating letter | Shows the tire’s speed category |
| DOT 2525 | Build date: week 25 of 2025 | Shows tire age, which matters on stored or used tires |
The Other Number Many Drivers Miss
Look for the DOT code near the bead area. The last four digits tell you when the tire was made. 2525 means the 25th week of 2025. That’s one of the handiest things on the whole sidewall, mostly when you’re buying a used tire or a new tire that may have sat in storage for a while.
The date does not mean the tire is bad once the calendar flips. It does mean you should judge age, tread depth, cracking, flat spots, and storage history as a package.
Check the build date before you buy old stock or used tires.
Grades You May See On Passenger Tires
Passenger tires sold in the United States often carry Uniform Tire Quality Grading, or UTQG, marks. These are separate from the size code. They tell you about treadwear, traction, and temperature resistance. NHTSA’s tire safety ratings page explains that these grades are used on passenger vehicle tires sold in the United States. They do not appear on every tire type, so don’t panic if you don’t see them on a winter tire or a temporary spare.
UTQG grades work best as comparison tools inside the same broad class of tire. A treadwear grade of 600 does not mean double the life of 300 in every street condition.
| Marking | Plain-English Meaning | What It Tells You |
|---|---|---|
| Treadwear 300 / 500 / 700 | A comparison grade for wear rate | Higher numbers usually point to longer wear in the test system |
| Traction AA / A / B / C | Straight-line wet braking grade | AA and A sit above B and C |
| Temperature A / B / C | Heat resistance grade | A handles heat better than B or C in the grading test |
| M+S | Mud and snow marking | Common on all-season tires |
| 3PMSF | Three-Peak Mountain Snowflake symbol | Shows the tire met a winter traction test |
| XL | Extra load tire | Built to carry more weight at higher inflation pressure |
How To Match The Numbers To Your Vehicle
The safest starting point is your driver’s door-jamb placard or owner’s manual. That sticker lists the original tire size and the inflation pressure your vehicle maker wants. Use the sidewall to confirm what is on the car now. Then match that against the placard before you buy anything.
When you compare options, check these points in order:
- Wheel diameter must match your wheel
- Load index should meet or beat the vehicle requirement
- Speed rating should meet the vehicle requirement
- Overall size should stay close to stock if you want the speedometer and clearance to stay sane
If you’re changing wheel size, keep the full package in range so the new tire clears the car, carries the weight, and suits the wheel width.
Common Mistakes People Make With Tire Numbers
A lot of mix-ups come from one bad habit: stopping at the width and wheel size. A tire marked 225/45R17 and another marked 225/65R17 fit the same wheel diameter, yet their sidewall height and overall diameter are miles apart. They won’t behave the same, and one may not fit the car at all.
Here are the slipups that show up most often:
- Buying by wheel size only
- Ignoring the load index on heavier crossovers and vans
- Missing the DOT build date on used tires
- Assuming every snow-looking symbol means the same winter grip
- Mixing up max sidewall pressure with the pressure your car needs day to day
Once you know where each number sits, those mistakes get easier to spot.
Read The Sidewall With Confidence
The numbers on a tire are not random. They’re a compact label that tells you fit, shape, load, speed class, and age. Read them left to right, and the code starts to make sense fast.
If you’re checking a tire in your driveway, start with the main size code, then read the load index, speed rating, and DOT date. That small habit can save you from buying the wrong tire, overpaying for old stock, or mounting a set that does not match your vehicle’s needs.
References & Sources
- Michelin.“How to Read Tire Markings and Sidewall Codes.”Explains the reading order for size, load, and speed markings on passenger tire sidewalls.
- National Highway Traffic Safety Administration (NHTSA).“Tire Safety Ratings and Awareness.”Explains UTQG tire ratings and where those sidewall grades apply on passenger vehicle tires sold in the United States.