What Are Measurements On A Tire Size? | Read Them Right

Tire size markings show width, sidewall height, construction type, wheel diameter, load index, and speed rating on the sidewall.

If the string of letters and numbers on a tire looks like a secret code, you’re not alone. A size such as P225/65R17 102H packs a lot into one line. It tells you the tire’s width, sidewall ratio, construction, wheel fit, load rating, and speed rating.

Once you know how to read that code, tire shopping gets less messy. You can spot when two tires are close but not the same and catch a mismatch before you buy.

What Are Measurements On A Tire Size? Start With The Main Code

Take P225/65R17 102H. Read it from left to right. Each part answers one fit or performance question.

The Prefix Tells You The Tire Type

The first letter is often a type marker. “P” means passenger vehicle tire. “LT” means light truck. Some tires start with no letter at all, which often points to a Euro-metric passenger tire. Trailer tires use “ST.” This part matters because two tires can share the same width and wheel diameter but still be built for different jobs.

The First Number Is Width

In 225/65R17, the 225 is the section width in millimeters. That is the tire’s width from sidewall to sidewall, not the tread width that touches the road. So a 225 tire is not 225 millimeters of tread. Raised ribs, shoulder shape, and rim width can shift the real mounted width a bit.

The Second Number Is Sidewall Height Ratio

The 65 is the aspect ratio. It means the sidewall height equals 65% of the tire’s width. On a 225 tire, that works out to a sidewall height of 146.25 millimeters. A smaller ratio, such as 55, gives the tire a shorter sidewall. A bigger ratio, such as 70, gives it a taller one.

This number trips people up because it is not a direct height measurement. It is a percentage. So the sidewall on a 255/55 tire is taller than the sidewall on a 225/55 tire while both share the same ratio.

The Letter In The Middle Shows Construction

The “R” stands for radial construction, which is what nearly all modern passenger tires use. Older tires may show “D” for diagonal or bias construction. On most cars, you’ll almost always see R.

The Last Number In The Main Size Is Wheel Diameter

The 17 means the tire fits a 17-inch wheel. This is the wheel diameter, not the tire’s full outside diameter. A 225/65R17 and a 225/55R17 both fit a 17-inch wheel, but they do not have the same overall height.

The Final Digits And Letter Are Service Marks

The 102 is the load index. The H is the speed rating. These sit after the main size because they are about carrying weight and handling speed, not the tire’s physical dimensions. Still, they matter when you replace a tire. Michelin’s load and speed rating page lays out why those marks should match the vehicle’s requirements.

Here’s a compact way to read the full code:

• P = Passenger tire
• 225 = Section width in millimeters
• 65 = Sidewall height as a percent of width
• R = Radial construction
• 17 = Wheel diameter in inches
• 102H = Load index and speed rating

Marking	What It Means	What To Watch
P	Passenger vehicle tire	Not the same as LT or ST tires
LT	Light truck tire	Built for heavier duty use and different load rules
225	Section width in millimeters	Measures sidewall to sidewall, not tread width
65	Aspect ratio	A percent, not a direct height number
R	Radial construction	The common build on modern road tires
17	Wheel diameter in inches	Must match the wheel exactly
102	Load index	Shows how much weight one tire can carry
H	Speed rating	Shows the tire’s rated top speed class
XL	Extra load tire	May carry more load at a higher pressure

Tire Size Measurements People Mix Up Most Often

Most mistakes happen when drivers treat all numbers on the sidewall as size. They’re not. Some marks tell you fit. Others tell you capacity, age, weather use, or factory testing grades.

Width And Tread Width Are Not The Same Thing

When a tire says 225, that is the section width. The tread is often narrower. So if you compare one brand’s 225 with another brand’s 225, the shoulder shape can make them look different even when the listed size matches.

Aspect Ratio Is Not Overall Tire Height

A 65-series tire is not 65 millimeters tall. It means the sidewall height is 65% of the width. To get rough outside diameter, take two sidewalls, convert them to inches, and add wheel diameter.

Wheel Diameter Is Not Tire Diameter

This is the one that causes the most bad purchases online. A 17-inch wheel can take many tire sizes, each with a different outside diameter. Changing that outside diameter can alter speedometer accuracy, gearing feel, and wheel-well clearance.

Load Index Is Not Optional

People often match only the width, ratio, and rim number. That’s not enough. A replacement tire should meet the vehicle’s required load and speed marks. If the car came with a 102H tire, dropping to a lower load or speed class can create a bad fit on paper even if the tire mounts fine.

If you want a manufacturer walk-through of the basic size code, Bridgestone’s tire size explanation is a handy cross-check.

Other Sidewall Numbers That Are Not The Size

A tire sidewall holds more than the size code. Some of these marks help with buying, and some help with maintenance.

DOT Code And Build Date

The DOT string includes the tire identification number. The last four digits show the week and year the tire was made. A code ending in 1225 means the tire was built in the 12th week of 2025. That number is about age, not size.

Maximum Load And Maximum Pressure

You’ll also see wording for maximum load and maximum inflation pressure. Those are tire limits, not your daily pressure target. The pressure you should run is usually on the vehicle placard inside the driver’s door area, not the tire sidewall.

UTQG Marks

Many passenger tires also show treadwear, traction, and temperature grades. Those can help when you compare models, but they still do not tell you whether the tire fits your wheel.

Seasonal And Special Marks

You may spot M+S, a three-peak mountain snowflake symbol, Run Flat wording, or directional arrows. Those marks tell you about weather use or special design traits. They sit alongside the size, not inside it.

Sample Size	What Changes	What Stays The Same
225/65R17	Taller sidewall and taller overall tire	Fits a 17-inch wheel
225/55R17	Shorter sidewall and shorter overall tire	Fits a 17-inch wheel
245/65R17	Wider tire and taller sidewall	Fits a 17-inch wheel
225/65R18	Needs a larger wheel	Same section width and ratio
LT245/75R17	Truck-type casing and load rules	Fits a 17-inch wheel

How To Check The Right Tire Measurement For Your Vehicle

The sidewall on your current tire is a strong starting point, but it should not be your only one. Cars get fitted with non-stock wheels and odd replacement sizes all the time. If you want the size the vehicle was built around, check these spots in order:

• The tire placard on the driver’s door jamb
• The fuel door on some vehicles
• The owner’s manual
• The current tire sidewall, if you know the setup is correct

That check matters most when you’re replacing one tire on an all-wheel-drive vehicle, changing wheel size, or buying from a thin marketplace listing. One wrong digit can turn a clean match into rubbing or an off speedometer.

If Your Car Has Aftermarket Wheels

Check the actual wheel diameter and width before you order tires. Used cars often wear wheels that differ from the factory setup, so the current tire size may fit the car but not match the door placard.

The Sidewall Tells You More Than Most Drivers Realize

Tire size measurements are not random. They follow a set order: tire type, width, sidewall ratio, construction, wheel diameter, then service marks. Once you can read that line, you can tell whether two tires are true matches or just close enough to fool the eye.

The best habit is simple. Read the full sidewall code, not just the width and wheel diameter. Match the type, the ratio, the rim size, the load index, and the speed rating against the vehicle placard. Do that, and tire shopping gets a lot less guessy.