How to Read a Tire Size: The Complete Guide (2026)

Q: What does the R mean in a tire size?

The 'R' stands for radial construction, which describes how the tire's internal plies are arranged. Nearly all modern passenger and light truck tires use radial construction.

Q: Can I use a different tire size than what came on my car?

Minor size changes can sometimes work, but you should always confirm fitment with your owner’s manual and a tire professional. Significant changes can affect speedometer accuracy, handling, brake performance, and safety systems.

Q: What is the load index on a tire?

The load index is a numeric code showing the maximum weight a tire can support when properly inflated. You should always match or exceed your vehicle’s required load index.

Q: What does the speed rating on a tire mean?

The speed rating is a letter that indicates the maximum sustained speed the tire is designed for. Never choose a tire with a lower speed rating than your vehicle requires.

Decoding a tire size in 60 seconds

The string of numbers and letters on your tire sidewall, like 265/70R17 121S, tells you everything about what that tire is designed to do. Once you understand the code, you can confidently compare tires, verify fitment, and avoid costly mistakes.

The full tire code breakdown

Example tire size

265/70R17 121S

Section width: 265; millimeters
Aspect ratio: 70; % of width
Construction: R; radial
Rim diameter: 17; inches
Load index: 121; 3,197 lbs per tire
Speed rating: S; 112 mph

Note on the prefix

Some tires begin with a letter prefix: P (Passenger), LT (Light Truck), or ST (Special Trailer). No prefix means the tire meets Euro metric standards. LT tires have higher load ratings and are common on trucks and tow vehicles.

Section width: the first number (265)

265Measured in millimeters

The width of the tire from sidewall to sidewall at its widest point
Common passenger widths range from 185mm to 295mm
Wider tires provide more grip but can reduce fuel economy and increase road noise
This is the tire’s width, not the tread contact width, which is narrower

Example: A 265mm-wide tire is about 10.4 inches across. Going from 265 to 275 adds roughly 0.4 inches (10mm) of width per tire.

Aspect ratio: the second number (70)

70Measured in % of width

The sidewall height as a percentage of the tire’s section width
Example: 265 × 0.70 = 185.5mm of sidewall height
Lower numbers (35–55) = low-profile, sportier look, stiffer ride
Higher numbers (65–80) = more sidewall, better cushioning, common on trucks and SUVs

The 70 means the sidewall is 70% as tall as the tire is wide.

265 mm wide
Sidewall: 70% of 265 = 186 mm tall
R17: fits a 17-inch wheel

The 265/70R17 from this guide, drawn to scale. A lower aspect ratio, such as 45, means a shorter sidewall on the same width.

Changing aspect ratio changes the overall tire diameter. Going too wide or too tall can cause rubbing and throw off your speedometer.

Rim diameter: the number after R (17)

17Measured in inches

The diameter of the wheel the tire is designed to fit, measured in inches
Must match your wheel exactly: a 17-inch tire will not fit a 16 or 18-inch wheel
Common sizes: 15, 16, 17, 18, 19, 20 and 22 inches
Larger diameter wheels typically require lower aspect ratio tires to keep the same overall diameter

This is the one number you cannot fudge. Always match rim diameter exactly.

Load index and speed rating (121S)

Load index (121)

A numeric code representing the maximum weight the tire can support when properly inflated. Always match or exceed your vehicle's required load index.

Example load index values and their maximum load per tire
Index	Max load
95	1,521 lbs
100	1,764 lbs
110	2,337 lbs
121	3,197 lbs
128	3,748 lbs

The full chart is in Tire load index explained.

Speed rating (S)

A letter indicating the maximum sustained speed the tire is designed for. Never use a tire rated lower than your vehicle requires.

Common speed ratings, their maximum speeds and typical use
Rating	Max speed	Common use
S	112 mph	Trucks, SUVs, family cars
T	118 mph	Family cars, minivans
H	130 mph	Sport sedans, coupes
V	149 mph	Sports cars
W / Y	168 / 186 mph	High-performance cars

More ratings in the speed rating chart.

Where to find your required tire size

Driver's door jamb

Most common location. Open the driver's door and look for the sticker on the door frame. Lists the OEM recommended tire size and inflation pressure.

Owner's manual

The specifications section lists the original equipment tire size plus acceptable alternatives for different trim levels.

Fuel door or glove box

Some vehicles print tire specs on the inside of the fuel door or glove box lid, especially trucks with dual load ratings.

Already know your size? You can browse tires by size or see every tire in 265/70R17. To check fit for a specific trim, search by your vehicle instead.

Frequently asked questions

What does the R mean in a tire size?

'R' stands for Radial, describing how the tire's internal cord plies are arranged radially from the center. Nearly all modern passenger and light truck tires are radial construction.

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

Minor variations are sometimes acceptable, but you should always check your owner's manual. Significant changes can affect speedometer accuracy, handling, braking, and safety systems like ABS.

What is the load index on a tire?

The load index is a numeric code that corresponds to the maximum weight a tire can support. A load index of 95 = 1,521 lbs max. Always match or exceed your vehicle's required load index.

What does the speed rating on a tire mean?

The speed rating is a letter indicating the maximum sustained speed the tire is designed for. H = 130 mph, V = 149 mph, W = 168 mph. Never use a tire with a lower speed rating than required.

What is UTQG and do I need to care about it?

UTQG stands for Uniform Tire Quality Grading, a US government rating system with three components: Treadwear (higher = lasts longer), Traction (AA, A, B, or C for wet braking), and Temperature (A, B, or C for heat resistance). It's useful for comparing tires, but real-world performance varies by brand.

How to read a tire size