How To Read Tire Markings | Decode Sidewall Numbers

A tire sidewall shows size, load, speed, construction, and age, so you can pick the right fit and spot red flags before you buy.

A sidewall code like P225/65R17 102H looks dense at first glance. Read it in order and it turns into a plain checklist: vehicle type, width, profile, construction, wheel size, load class, speed class, and age.

That’s why learning How To Read Tire Markings pays off. It helps you skip wrong-size orders, mixed service types, and old stock that looks fresh from ten feet away.

How To Read Tire Markings On A Passenger Tire

Start with a common sample: P225/65R17 102H. Each piece means something different.

• P = passenger vehicle service type
• 225 = section width in millimeters
• 65 = aspect ratio
• R = radial construction
• 17 = wheel diameter in inches
• 102 = load index
• H = speed rating

Read left to right and you’ll know whether the tire fits the wheel, what shape it has, how much weight it can handle, and what speed class it belongs to.

What The First Letter Means

The opening letter shows service type. P means passenger. LT means light truck. ST means special trailer. T marks a temporary spare. That first code matters because those tires are built for different jobs.

A trailer tire marked ST is built for trailer axles, not for the front or rear of a passenger vehicle. A T temporary spare is only there to get you off the roadside and to a repair shop. Those markings save you from one of the easiest tire-buying mistakes: choosing something that fits the wheel but not the job.

Some passenger tires start with a number and skip the opening P. That is common on Euro-metric sizes. The rest of the size still reads in the same order, so 225/45R18 95W still gives width, aspect ratio, construction, rim size, load index, and speed rating.

Width, Aspect Ratio, And Construction

The three-digit number after the first letter is width in millimeters. In a P225 tire, that width is 225 mm. The next number is the aspect ratio. A 65 means the sidewall height is 65 percent of the width. Lower numbers usually mean a shorter sidewall. Higher numbers mean a taller one.

Then comes the construction code. On most modern road tires, you’ll see R for radial. Older or niche tires may show D, B, or a dash mark for bias construction.

Rim Size, Load Index, And Speed Rating

The next number is wheel diameter in inches. A 17 fits a 17-inch wheel only. After that comes the load index, which is a coded number tied to a load chart. The last letter is the speed rating, a tested class marker rather than a driving target.

Take another sample: 235/45R18 98Y XL. Here, the tire has no opening P, fits an 18-inch wheel, carries a 98 load index, uses a Y speed class, and adds XL to show an extra-load setup.

Other Sidewall Codes That Matter

Once the size string makes sense, scan the rest of the sidewall. These smaller labels tell you about weather use, extra-load setup, mounting direction, and wear grades.

The basics of tire size markings stay consistent across many passenger and light-truck tires, even when brands place the text in different spots.

Marking	Meaning	Check This
P / LT / ST / T	Service type	Match the tire to the vehicle’s job
225 / 245 / 275	Section width	Stay within approved fitment
65 / 55 / 40	Aspect ratio	Shorter or taller sidewall changes feel and clearance
R / D / B	Construction	Radial is common on modern road vehicles
17 / 18 / 20	Wheel diameter	Must match the wheel exactly
91 / 102 / 121	Load index	Meet the vehicle requirement
T / H / V / W	Speed rating	Match or exceed the required class
XL / M+S / 3PMSF / ROTATION	Extra load, winter, or mounting label	Use the right setup for season and installation

XL, Load Range, And Winter Symbols

XL means extra load. On truck tires, you may also see a load range letter such as C, D, or E. M+S marks a mud-and-snow tire. The three-peak mountain snowflake, shortened to 3PMSF, marks a tire that passed a snow-traction test and is the stronger winter clue.

Directional And Asymmetrical Labels

If the sidewall shows ROTATION with an arrow, the tire is directional and must spin that way once mounted. If it shows OUTSIDE or INSIDE, the tread is asymmetrical and each side has a set position. Mounting either style the wrong way can hurt water evacuation, noise control, and grip.

UTQG Grades

On many passenger tires sold in the United States, you’ll also see UTQG grades for treadwear, traction, and temperature. They help compare tires within that system, but they do not cover every tire category.

DOT Code, Max Pressure, And Tire Age

This is where many drivers get tripped up. The max pressure molded into the sidewall is not the normal inflation target for daily use. Use the vehicle placard on the driver’s door jamb or the owner’s manual. The NHTSA TireWise page says the correct pressure comes from the vehicle maker, not the sidewall maximum.

The sidewall also lists a max load figure. Read that as a molded limit for that tire in that setup, not as a hint that your vehicle should be loaded right up to the edge. Your vehicle still has its own axle and gross-weight limits.

Then find the DOT code. The last four digits of the Tire Identification Number show the week and year the tire was made. A code ending in 1225 means the 12th week of 2025. On some tires, the full DOT string appears on only one sidewall, so check both sides if needed.

That date matters because rubber ages with time and heat cycles. A tire with deep tread can still be a poor buy if it has been sitting for years.

Code	Plain Meaning	What To Do
102H	Load index 102, speed rating H	Check both parts together
XL	Extra-load tire	Use the right inflation spec for that setup
M+S	Mud-and-snow marking	Common on many all-season tires
3PMSF	Severe-snow symbol	Stronger winter clue than M+S alone
OUTSIDE / ROTATION	Mounting instruction	Make sure the tire is installed the right way
1225	12th week of 2025	Check age before install

Mistakes That Lead To The Wrong Tire

One common mistake is reading only the width and wheel diameter, then stopping there. A P225/65R17 and an LT225/65R17 may look close on paper, yet they are not the same tire in use. Load index, speed class, service type, and extra-load markings all matter.

Another miss is filling to the sidewall max instead of the door-sticker spec. Then there’s age. Fresh-looking tread can fool you, but the DOT date settles that question fast.

Match The Sidewall To The Door Sticker

When you replace tires, compare the sidewall with the tire placard on the driver’s door jamb. That placard gives the approved size and inflation target. If you want to change wheel size, check load capacity, clearance, and overall diameter before buying.

If your vehicle uses different sizes front and rear, copy each axle’s spec exactly unless you are working from an approved alternate setup. Mixing a front size with a rear size just because both clear the fender is a quick way to create rubbing, dash warnings, or uneven handling.

A Fast Check In Person

• Read the full size string.
• Match wheel diameter exactly.
• Check load index and speed rating.
• Look for XL, winter symbols, or mounting labels.
• Find the DOT date code.
• Use the door-jamb placard for inflation.

Once you know that order, the sidewall stops looking cryptic. It turns into a clean buying checklist that helps you choose the right tire with less guesswork.