How To Read A Tire Size Number | Decode Sidewall Marks

A tire code shows width, sidewall ratio, construction, wheel diameter, load index, and speed rating in one line.

Most drivers see a code like 225/45R17 94V and freeze for a second. It looks like shop jargon. It isn’t. That line is just a compact label that tells you how the tire is built and what kind of wheel and workload it was made for.

Once you break the code into chunks, the sidewall gets easy to read. You can tell whether a tire is narrow or wide, short or tall, made for a passenger car or a light truck, and whether it fits your wheel at all. That makes replacement shopping less of a coin toss.

You don’t need a chart for every part, either. Most of the string follows a fixed order. Read it left to right, and each chunk answers one plain question: what vehicle class, how wide, how tall, what construction, what rim size, how much load, and what speed class.

How To Read A Tire Size Number On A Passenger Tire

Take this sample: P225/45R17 94V. Read it in seven bites, not one gulp.

• P = passenger vehicle tire
• 225 = width in millimeters
• 45 = aspect ratio
• R = radial construction
• 17 = wheel diameter in inches
• 94 = load index
• V = speed rating

That’s the whole pattern. Not every tire starts with a letter, and some truck or trailer tires use a different prefix, but the middle of the code stays familiar. Once your eye spots the slash and the letter after it, the rest falls into place.

Read The Prefix First

The first letter tells you what class the tire was built for. P means passenger. LT means light truck. ST marks trailer tires. T is often used for a temporary spare. If there’s no prefix, many metric tires still follow the same width, ratio, construction, and diameter pattern.

This letter matters when you replace tires. A light-truck tire is built for a different duty than a passenger-car tire, even when the rest of the size looks close.

Then Read The Width

The first three-digit number is the tire’s section width in millimeters. In our sample, 225 means the tire is about 225 mm wide at its widest point when mounted and inflated to spec.

A bigger number usually means a wider tire. Wider tires can change steering feel, wet grip, fender clearance, and fuel use. That’s why matching the placard size matters more than picking a number that “looks close.” The NHTSA tire page says replacement tires should match the size recommended by the vehicle maker.

Next Comes The Aspect Ratio

The two-digit number after the slash shows sidewall height as a percentage of the width. In 225/45, the sidewall height is 45% of 225 mm. A lower ratio means a shorter sidewall. A higher ratio means a taller one.

Shorter sidewalls tend to look sportier and feel tighter over smooth pavement, yet they also leave less cushion over potholes. Taller sidewalls usually ride softer and give the wheel more protection from curb rash.

Read The Construction Letter

R means radial construction, which is what you’ll see on most modern passenger tires. You may also run into D for diagonal or bias construction on some specialty tires. On normal road cars, radial is the usual mark.

Read The Wheel Diameter

The number after the construction letter is the wheel diameter in inches. In R17, the tire fits a 17-inch wheel. This part is non-negotiable. A 17-inch tire does not fit an 18-inch wheel, even if the rest of the code looks close enough to tempt you.

Finish With Load Index And Speed Rating

The last numbers and letter tell you how much weight the tire can carry and the speed class it belongs to when properly inflated. 94V means load index 94 and speed rating V. The number is not pounds by itself. It points to a rating chart.

When you shop, don’t drop below the vehicle maker’s required load index or speed rating unless the maker gives another approved spec. That’s one of the easiest spots to miss when two tires share the same width and rim diameter.

What The Code Means At A Glance

Here’s the sidewall broken into a table you can scan in seconds. Use it when you’re standing in a garage, on a used-car lot, or staring at a tire listing online.

Code Part	What It Means	What To Check
P / LT / ST / T	Vehicle or tire class	Match the tire type to the vehicle and job
225	Section width in millimeters	Too wide can rub; too narrow can change feel
45	Aspect ratio, or sidewall height as a share of width	Lower ratio rides firmer; higher ratio rides softer
R	Radial construction	Most passenger vehicles use radial tires
17	Wheel diameter in inches	Must match the wheel exactly
94	Load index	Do not go below the required rating
V	Speed rating	Choose a rating that meets the vehicle spec
DOT + 4 digits	Plant code and build date	Use the last four digits to read week and year

Where People Trip Up When Reading Tire Sizes

Most mix-ups happen in three spots. The first is width versus diameter. A tire can share a width with another tire and still fit a different wheel. The second is aspect ratio. A 225/45R17 and 225/55R17 fit the same wheel diameter, yet the second tire stands taller overall. The third is load and speed. Those last characters matter just as much as the size block in the middle.

Another snag is assuming the door placard, the current tire, and the online listing all tell the same story. They should, but used cars and secondhand wheels can muddy the picture. If the tire on the car differs from the placard, trust the placard or owner’s manual first, then check with the vehicle maker or a tire shop before buying.

Do Not Skip The DOT Date Code

Sidewalls hold more than size data. They also carry the DOT tire identification number. The last four digits show the week and year the tire was made. A code ending in 2324 means the 23rd week of 2024. The NHTSA Tire Buyers’ FAQ points out that those last four digits tell you the build date.

That date does not tell you whether a tire is good or bad by itself, but it does tell you how old it is. Age matters when you’re buying a spare, shopping a warehouse deal, or checking a used car that sat for long stretches.

Common Tire Size Examples Decoded

These examples show how small changes in the code change the tire you end up with.

Example Size	Plain-English Read	What Stands Out
205/65R16 95H	205 mm wide, taller sidewall, 16-inch wheel	Common on sedans and small crossovers
225/45R17 94V	225 mm wide, shorter sidewall, 17-inch wheel	Sharper look and less sidewall height
235/55R18 100H	235 mm wide, mid-height sidewall, 18-inch wheel	Often balances ride comfort and wheel size
LT265/70R17	Light-truck tire, 265 mm wide, tall sidewall	Built for truck duty, not a plain passenger swap
ST205/75R15	Special trailer tire, 205 mm wide, 15-inch wheel	Trailer code, not for drive or steer axles on cars

How To Match A New Tire Without Guessing

If you’re buying replacements, don’t read the tire in isolation. Cross-check three places:

1. The driver’s door placard
2. The owner’s manual
3. The sidewall of the current tire, if you know it’s the right fit

If all three agree, you’re on solid ground. If one disagrees, slow down. A previous owner may have changed wheel size, fitted a plus-size package, or mounted the wrong tires. That’s when sidewall reading pays off. You can spot the mismatch before money leaves your wallet.

Also watch the service description, which is the load index and speed rating paired together near the end of the size. Two tires can look alike at first glance yet differ on that last chunk. That can change how much weight the tire can carry and what speed class it was built to handle.

What To Ignore And What To Read

Don’t get sidetracked by branding, marketing names, or giant sidewall lettering. They grab your eye, but they don’t tell you fit. Read the size line, the service description, the DOT date, and the tire type first. Those marks answer the fit and safety questions.

After that, you can read the rest of the sidewall if you want more detail. You may see M+S or the three-peak mountain snowflake mark for winter use, plus UTQG grades for treadwear, traction, and temperature on many passenger tires sold in the United States.

Reading Tire Size Numbers Gets Easier Fast

The trick is to stop seeing one long code. Split it into pieces. Prefix, width, ratio, construction, diameter, load index, speed rating. Once you do that two or three times, the sidewall stops feeling cryptic.

If you only want the bare reading order, use this: letter, width, slash ratio, construction letter, wheel diameter, load number, speed letter, then DOT date. That short pattern is enough for most tire-shopping trips and garage checks.

A tire sidewall is blunt. It tells you what fits, what the tire was built to carry, and when it was made. Read that line well, and you’ll shop smarter, catch bad matches faster, and spend less time second-guessing the code in front of you.