How to Read Tire Size (Simple Beginner Guide)

Marko S.
Dec 3
3 min read

Tire sizes can look confusing at first — numbers, letters, slashes, and characters all packed into one line. But once you understand what each part means, reading a tire size becomes simple. This beginner-friendly guide breaks down everything in an easy, clear way.

Learning how to read your tire’s size is essential when choosing a new tire, measuring lift changes, or picking the correct spare tire cover.

The Basic Tire Size Format

Most tires follow this format:

Example:265/70R17

Each section means something specific:

265 → Tire width (millimeters)
70 → Aspect ratio (height % of width)
R → Radial construction
17 → Wheel diameter (inches)

Let’s break it down step by step.

Width (First Number)

In 265/70R17, the 265 is the tire’s width.

It represents:

The width in millimeters
Measured from sidewall to sidewall

Wider tires offer more grip and a bigger footprint; narrower tires improve efficiency.

Aspect Ratio (Second Number)

In 265/70R17, the 70 is the aspect ratio.

It means:

The sidewall height is 70% of the tire’s width

So:

265 mm width
70% of 265 = 185.5 mm sidewall height

Higher aspect ratio → taller sidewall

Lower aspect ratio → shorter, sportier sidewall

Construction Type (Letter After Aspect Ratio)

Most tires today use R, which stands for:

Radial construction

Other letters are rare:

D = Diagonal (bias-ply)
B = Belted

For modern vehicles, R is almost always the standard.

Rim Diameter (Last Number)

In 265/70R17, the 17 indicates the tire fits a:

17-inch wheel

Common wheel diameters include:

This number must match your vehicle’s wheel size exactly.

Additional Tire Markings You Might See

Besides the main size, tires include extra information. Here are the most common ones.

Load Index (e.g., 115)

A number that tells how much weight the tire can carry.

Higher load index → stronger tire.

Speed Rating (e.g., S, T, H, R)

Letter that indicates max safe speed.

Examples:

S = up to 112 mph
T = up to 118 mph
H = up to 130 mph

LT or P Prefix

You may see:

P = Passenger vehicle tire
LT = Light truck tire (stronger for towing/off-road)

Example: LT265/70R17 is a reinforced version of the standard 265/70R17.

XL (Extra Load)

Indicates reinforced sidewalls for heavier loads.

M+S (Mud and Snow)

Suitable for light winter conditions.

3PMSF (Three-Peak Mountain Snowflake)

Certified for true winter performance.

How Tire Sizes Convert to Overall Diameter (Inches)

You’ll often hear off-road drivers say:

“33s”
“35s”
“37s”

These are the tire’s overall diameter in inches, not the printed size.

For example:

285/70R17 ≈ 33 inches
315/70R17 ≈ 35 inches
37x12.50R17 = 37 inches

Knowing this helps when selecting the correct spare tire cover.

Where to Find Your Tire Size

Check these locations:

Outer sidewall of the tire
Vehicle door jamb sticker
Owner’s manual
Inside the fuel door (some models)

Rear-mounted spares may not match the other four tires — always check the tire that’s actually on the spare wheel.

Why Knowing Your Tire Size Matters

Understanding your tire size helps with:

Buying new tires
Ordering wheel packages
Selecting a spare tire cover
Choosing lift kits or suspension parts
Matching spare tire sizes
Maintaining proper gearing and speedometer accuracy

For rear-mounted spares, the correct cover size depends directly on your tire’s printed size.

Final Thoughts: Tire Sizes Are Easy Once You Know the Basics

Reading tire size numbers may look confusing at first, but once you understand width, aspect ratio, and rim size, everything becomes simple.

Whether you're buying new tires or protecting your spare with a high-quality SpareCover®, knowing your exact tire size ensures a perfect fit.