Where is the tire size on my car?

Two places: the sidewall of the current tire, and a sticker on the driver-side door jamb. The door-jamb sticker is the factory-recommended size and is the safer source if your tires were swapped to a non-stock size by a previous owner.

What does R mean in 245/40R18?

R stands for radial construction, meaning the cord plies run perpendicular to the direction of travel. Almost every passenger-vehicle tire sold today is radial. The number after R is the wheel rim diameter in inches.

Can I use a different tire size than what is on my car?

Only if the alternate size has the same overall diameter (within 3 percent), the same or higher load index, and an equivalent speed rating. Mismatching size affects speedometer accuracy, ABS, AWD systems, and suspension geometry. Stick to the door-jamb spec unless you have done the math.

What does the DOT date code mean?

The 4-digit code at the end of the DOT stamp is the production week and year. 2123 means the 21st week of 2023. Tires older than 6 years on the shelf can be safe but the rubber has aged. Most manufacturers recommend replacing tires by 10 years old regardless of tread depth.

What does UTQG tell me?

Uniform Tire Quality Grading is a US Department of Transportation rating: treadwear (higher is longer-lasting), traction (AA, A, B, C wet grip), temperature (A, B, C heat resistance). Treadwear is comparable only within one brand, since each tester can pick its own baseline.

How to Read Tire Size: Sidewall Numbers Explained

TL;DR

A code like 245/40R18 97Y tells you everything: 245 mm wide, 40 percent aspect ratio, radial, 18 inch rim, 97 load index, Y speed rating.
Match the door-jamb sticker, not the previous owner’s choice.
Load index and speed rating must equal or exceed the original spec, never go lower.
The 4-digit DOT date code tells you when the tire was made (week and year).
UTQG numbers compare tires within one brand only, not across brands.

The size code, decoded

The most important string on the sidewall looks like this: 245/40R18 97Y. Here is what each piece means.

245: section width

The first number is the section width in millimeters, measured at the widest point of the tire when it is mounted on the recommended rim and inflated. Wider tires have more rubber on the road and usually grip better in dry conditions, but they are noisier, heavier, and burn more fuel.

40: aspect ratio

The number after the slash is the aspect ratio: the height of the sidewall as a percentage of the section width. A 40 means the sidewall is 40 percent of 245 mm, or about 98 mm tall. Lower aspect ratios mean shorter sidewalls, sharper steering, and harsher rides.

R: construction

R is for radial. Almost every modern tire is radial. You may rarely see a D (bias-ply) on trailer tires.

18: rim diameter

The number after R is the wheel rim diameter in inches. This must exactly match your wheel.

97: load index

A coded number that maps to a maximum weight per tire. 97 means 1,609 lbs per tire; 100 means 1,764 lbs; 105 means 2,039 lbs. Always equal or exceed the door-jamb spec. Going below the original load index is unsafe and can void insurance.

Y: speed rating

Y means rated for sustained 186 mph. Other common ratings: H (130 mph), V (149 mph), W (168 mph). The rating is set by lab tests under load. Match or exceed the door-jamb spec.

The other markings

DOT code

Stamped on one sidewall: DOT XXXX XXXX 2125. The last 4 digits are the production week and year (2125 means week 21 of 2025). Most brands recommend replacing tires by their 10th birthday even if tread looks fine, because rubber compounds harden over time. NHTSA publishes guidance at nhtsa.gov/equipment/tires.

UTQG

Uniform Tire Quality Grading: three numbers/letters showing treadwear, traction, and temperature resistance. Treadwear of 500 means the tire wore at 5x the rate of a baseline reference tire on a government test. Compare only within one brand because each manufacturer picks its own reference.

Tread depth indicators

Small molded bars in the tread grooves at 2/32 of an inch. When the surrounding tread wears down to those bars, the tire is at the legal replacement threshold in most US states. We recommend replacing at 4/32 for wet weather safety.

M+S, 3PMSF

M+S means “mud and snow” (a mostly-marketing claim). 3PMSF (three-peak mountain snowflake) is the meaningful winter rating, based on a tested traction performance on packed snow. Required for true winter tires; see our all-season vs winter guide.

How to pick the right size

For 99 percent of drivers: match the door-jamb sticker exactly. That sticker is the manufacturer’s tested optimal size, balancing ride, fuel economy, suspension wear, and ABS calibration. Going up a rim size (plus-sizing) is fine if you keep the overall diameter the same and equal or exceed the load and speed ratings. If you are unsure, our shop-by-vehicle tool shows OEM-correct sizes for your year/make/model/trim.

Common mistakes

Mixing sizes: never run different sizes on the same axle. On AWD, never run different sizes anywhere on the car.
Ignoring load index on EVs and trucks: EVs and full-size SUVs need higher load indices because of battery and curb weight. The wrong tire will overheat and fail.
Buying old stock: always check the DOT date. Tires older than 6 years on the shelf are not new in any meaningful sense.
Trusting tread depth alone: a 6-year-old tire with deep tread can still be unsafe due to compound aging.

Buying with TireOrbit

Every tire on TireOrbit lists size, load index, speed rating, UTQG, and DOT date. Prices include a $130 install credit redeemed at your selected local installer, so the price you see is the price you pay. Browse all tires, search by size at by-size, or start with your vehicle.

FAQs

Where is the tire size on my car?: Two places: the sidewall of the current tire, and a sticker on the driver-side door jamb. The door-jamb sticker is the factory-recommended size and is the safer source if your tires were swapped to a non-stock size by a previous owner.
What does R mean in 245/40R18?: R stands for radial construction, meaning the cord plies run perpendicular to the direction of travel. Almost every passenger-vehicle tire sold today is radial. The number after R is the wheel rim diameter in inches.
Can I use a different tire size than what is on my car?: Only if the alternate size has the same overall diameter (within 3 percent), the same or higher load index, and an equivalent speed rating. Mismatching size affects speedometer accuracy, ABS, AWD systems, and suspension geometry. Stick to the door-jamb spec unless you have done the math.
What does the DOT date code mean?: The 4-digit code at the end of the DOT stamp is the production week and year. 2123 means the 21st week of 2023. Tires older than 6 years on the shelf can be safe but the rubber has aged. Most manufacturers recommend replacing tires by 10 years old regardless of tread depth.
What does UTQG tell me?: Uniform Tire Quality Grading is a US Department of Transportation rating: treadwear (higher is longer-lasting), traction (AA, A, B, C wet grip), temperature (A, B, C heat resistance). Treadwear is comparable only within one brand, since each tester can pick its own baseline.

Author

TireOrbit Editorial is our in-house team of former tire technicians, automotive journalists, and product engineers. We cite manufacturer specifications, NHTSA, and UTQG data.

How to read your tire size