How To Read Tire Size Chart | Decode Sidewall Numbers

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

If a tire sidewall looks like alphabet soup, you’re not alone. Most people see a code like 225/45R17 94V and stop at the wheel size. That misses half the story. Each number and letter tells you whether the tire will fit, how tall the sidewall is, how much weight the tire can carry, and the speed category it was built for.

Once you know the order, the code gets easy. You read it left to right, and each part answers one plain question: What vehicle type is this for? How wide is it? How tall is the sidewall? What wheel fits it? What load and speed does it handle? After that, you can match the sidewall to the door-jamb sticker and shop with a lot more confidence.

How To Read Tire Size Chart On Your Tire

Start with a real-world size: P225/45R17 94V. Read it in chunks, not as one long code. That makes the pattern stick.

1. P = passenger vehicle tire
2. 225 = tire width in millimeters
3. 45 = aspect ratio, or sidewall height as a percent of width
4. R = radial construction
5. 17 = wheel diameter in inches
6. 94 = load index
7. V = speed rating

That one line already tells you fit and a big part of performance. A wider tire can change grip and steering feel. A lower aspect ratio means a shorter sidewall. A bigger wheel diameter must match the wheel itself. Load index and speed rating must meet your vehicle’s requirements, not just “fit on the rim.”

Start With The Prefix

The first letter is often skipped, yet it matters. P means passenger tire. LT means light truck. Some tires have no prefix at all, which is common on many Euro-metric sizes. Temporary spares may use T.

Don’t swap classes just because the width and diameter match. A light-truck tire and a passenger tire can feel and behave differently under the same vehicle.

Read The Width And Aspect Ratio Together

The three-digit number is the section width in millimeters. In 225/45R17, the tire is 225 mm wide. The next number is the aspect ratio. Here, 45 means the sidewall height is 45 percent of the width.

That’s why two tires with the same width can still look different. A 225/65R17 has a much taller sidewall than a 225/45R17. Taller sidewalls usually ride softer. Shorter sidewalls often feel sharper in turns and show less flex.

Then Read Construction And Wheel Diameter

R stands for radial, which is what you’ll see on modern passenger vehicles. The number after it is the wheel diameter in inches. A 17-inch tire must go on a 17-inch wheel. No wiggle room there.

If you remember only one fit rule, make it this one: the tire’s diameter code and the wheel’s diameter must match exactly.

Reading Tire Size Numbers Before You Buy

The sidewall gives you the tire’s code, but your vehicle tells you which code it was built around. The safer tie-breaker is the placard on the driver’s door jamb or the owner’s manual. NHTSA’s tire safety brochure says replacement tires should match the original size or another size listed by the vehicle maker.

You can learn the rest of the molded markings from Michelin’s sidewall markings page, which lays out the order of width, aspect ratio, construction, diameter, load rating, speed symbol, pressure markings, and date code.

When you compare the sidewall with the placard, check more than size alone:

• Front and rear sizes may be different on some cars.
• Winter packages may use a different wheel and tire size.
• Load index and speed rating should meet the vehicle’s listed spec.
• “Max pressure” on the tire is not the same as your car’s cold tire pressure setting.

Code Part	What It Means	What To Check
P / LT / T	Tire class: passenger, light truck, or temporary spare	Match the vehicle type and spare setup
225	Section width in millimeters	Wider tires may need wheel and clearance checks
45	Aspect ratio, or sidewall height as a percent of width	Lower numbers mean shorter sidewalls
R	Radial construction	This is standard on modern road cars
17	Wheel diameter in inches	Must match the wheel exactly
94	Load index	Must meet or beat the vehicle spec
V	Speed rating	Must meet the maker’s requirement
XL / Reinforced	Higher-load version of that size	Needed on some heavier cars and EVs
DOT date code	Week and year of manufacture	Good to check before purchase or on older tires
M+S / 3PMSF	Snow-use markings	3PMSF is the stronger winter mark

What The Load Index And Speed Rating Tell You

The last part of the size line gets ignored all the time. That’s a mistake. In 94V, the 94 is the load index and V is the speed rating. The load index is a code tied to a weight value for one tire at the proper inflation level. The speed symbol is the tire’s speed category under set test conditions.

You don’t need to memorize the full chart to shop well. You just need to know that these codes are not decoration. Two tires can share the same 225/45R17 size and still differ in load index, speed rating, or both.

That matters most on heavier cars, crossovers, EVs, and performance models. If your door placard or owner’s manual calls for a higher service description, dropping lower can leave you with a tire that fits the wheel but falls short of the vehicle’s spec.

Why XL And Reinforced Marks Matter

An XL or reinforced tire can carry more load than a standard-load tire in the same size. You’ll often see that on cars with firmer setups, higher curb weight, or larger wheel packages. If the original tire was XL, match it unless the vehicle maker lists another approved option.

Marks That Sit Near The Size Code

Not every sidewall mark is part of the size line, yet many sit close enough to cause mix-ups. These are the ones worth spotting right away:

• Max load: the top load the tire can carry at its stated max pressure
• Max pressure: the tire’s upper pressure marking, not the car’s daily setting
• M+S: mud-and-snow tread design mark
• 3PMSF: mountain snowflake winter mark
• DOT: regulatory code that includes the build date

The daily inflation target still comes from the vehicle placard, not from the tire’s sidewall maximum. That single mix-up causes a lot of bad pressure checks.

Example Size	Plain-English Read	What Changes
205/55R16	205 mm wide, medium sidewall, 16-inch wheel	Common balance of ride and steering feel
225/45R17	Wider tire, shorter sidewall, 17-inch wheel	Sharper response, less sidewall height
235/65R17	Wide tire with tall sidewall, 17-inch wheel	More sidewall height, often seen on SUVs
LT265/70R18	Light-truck tire, wide, tall sidewall, 18-inch wheel	Built for truck-type duty and load needs
T125/70D17	Temporary spare, narrow, compact build	Spare-only tire, not a full-time match

Common Mistakes When Reading A Tire Size

Mixing Up Tire Width And Tread Width

The sidewall width number is section width, not the tread width you see touching the road. Actual tread width varies by tire model, even within the same size.

Treating Max Pressure As Daily Pressure

The sidewall’s max pressure marking is not your fill target for normal driving. The car maker’s placard is the daily reference for cold tire pressure.

Ignoring Staggered Setups

Some cars run one size in front and another in back. If you read only one tire and order four, you can end up with the wrong set.

Buying By Diameter Alone

A 17-inch tire still needs the right width, aspect ratio, load index, and speed rating. Diameter is just one piece.

A Five-Step Check Before You Order

1. Read the full size code on your current tire.
2. Check the driver-door placard for the factory size and pressure.
3. Match the wheel diameter exactly.
4. Match or beat the listed load index and speed rating.
5. Double-check front and rear sizes if the car uses a staggered setup.

Once you do that, a tire size chart stops feeling cryptic. It becomes a quick fit check. Read left to right, match the placard, and don’t skip the service description at the end. That’s the cleanest way to choose a tire that fits your wheel, suits your vehicle, and avoids costly ordering mistakes.