How to Read Tire Size: A Complete Technical & Practical Guide

March 5, 2026

If you’ve ever looked closely at your vehicle’s tires, you’ve probably noticed a series of letters and numbers printed on the sidewall. At first glance, these markings—such as 205/55R16 91V—may seem confusing or technical.

However, they actually provide essential information about your tire’s size, construction, load capacity, and performance capabilities. Understanding these markings is important when replacing tires, checking compatibility, or ensuring your vehicle maintains safe and efficient performance on the road.

In this complete guide, we will break down the tire size code step by step and explain what each number and letter means.

How to Read Tire Size?

Every tire sidewall contains standardized codes that help drivers and technicians identify the correct tire for a specific vehicle. These markings reveal critical details such as the tire’s width, sidewall height, wheel diameter, maximum load capacity, and speed rating. They also include additional indicators about the tire’s intended use, such as whether it is designed for passenger vehicles, light trucks, or winter conditions.

Learning how to interpret these codes makes it easier to choose the right replacement tires and maintain optimal vehicle safety.

At first glance, a tire size such as: P225/65R17 102H may look like a random string of numbers and letters.

In reality, it is a standardized engineering code that communicates:

Tire dimensions
Construction type
Load-carrying capacity
Maximum speed capability
Intended vehicle category

Understanding this code helps ensure:

Proper vehicle handling
Safety compliance
Accurate speedometer readings
Optimal fuel efficiency
Correct replacement selection

Let’s break it down the same way professional tire manufacturers explain it.

Step 1: Understanding the Full Tire Size Code

We’ll use this example:

P225/65R17 102H

Each section has a specific technical meaning.

1. Tire Type Prefix (P / LT / ST / T)

The first letter identifies the category of tire.

Tire Type Prefix (P / LT / ST / T)	Type of Vehicle	Features
P = Passenger Tire	Sedans Hatchbacks Crossovers Small SUVs	Ride comfort Low road noise Fuel efficiency
LT = Light Truck Tire	Pickup trucks Larger SUVs Vehicles carrying heavy loads	Reinforced sidewalls Higher load capacity Stronger internal construction
ST = Special Trailer	Used only for trailers — never for passenger vehicles.	High load-bearing capacity Stability Durability
T = Temporary Spare	Compact spare tires intended for short-term emergency use only.

2. Tire Width (225)

The number 225 represents the tire’s section width in millimeters.

This measurement is taken:

From the widest point of the outer sidewall
To the widest point of the inner sidewall
When properly mounted and inflated

So, this tire is 225mm wide.

Why Does Width Matters?

Width affects:

I. Road Contact Patch

Wider tires increase surface contact with the road, improving:

Cornering grip
Stability at high speeds

II. Fuel Economy

Narrower tires reduce rolling resistance, which:

Improves fuel efficiency
Reduces engine strain

III. Hydroplaning Resistance

Extremely wide tires may struggle more in deep water if tread design isn’t optimized.

3. Aspect Ratio (65)

The aspect ratio represents the sidewall height as a percentage of tire width.

Here:

65% of 225 mm = approximately 146 mm sidewall height.

Why Is It Important?

Aspect ratio directly impacts:

Ride Comfort

Higher aspect ratio (65–75):

Taller sidewall
Better shock absorption
Smoother ride

Lower aspect ratio (35–50):

Shorter sidewall
Sportier handling
More responsive steering

4. Construction Type (R)

The letter R stands for Radial construction.

In radial tires:

Internal cords run perpendicular to the direction of travel
Steel belts are layered beneath the tread

Radial tires provide:

Improved tread life
Better fuel efficiency
Enhanced road contact
Greater durability

Nearly all modern passenger vehicles use radial tires.

Older construction types include:

D (Diagonal/Bias-ply)
B (Belted bias)

5. Wheel Diameter (17)

This number indicates the diameter of the wheel rim in inches.

A tire marked R17 must be mounted on a 17-inch wheel.

This measurement:

Is always in inches
Must match exactly

You cannot mount:

A 17-inch tire on a 16-inch rim
Or vice versa

6. Load Index (102)

The load index is a numerical code that corresponds to the maximum weight the tire can safely carry.

For example:

102 = 850 kg per tire

Multiply by four tires:

850 × 4 = 3,400 kg maximum vehicle support (approximate total)

Why Load Index Is Critical?

Using a tire with a lower load index than recommended may cause:

Tire overheating
Sidewall failure
Blowouts
Reduced braking performance

Always match or exceed the manufacturer’s recommendation.

7. Speed Rating (H)

The final letter indicates the maximum speed capability under ideal conditions.

Common ratings:

Rating	Max Speed
T	190 km/h
H	210 km/h
V	240 km/h
W	270 km/h
Y	300 km/h

Important:
The speed rating reflects the tire’s ability to handle heat buildup at sustained speeds, not just top speed capability.

Understanding Additional Tire Sidewall Markings

Manufacturers also include other essential markings.

M+S (Mud and Snow)

This symbol indicates the tire meets basic all-season traction requirements.

Three-Peak Mountain Snowflake (3PMSF)

This symbol means:

The tire meets severe snow performance testing standards
It provides superior winter traction

DOT Code (Department of Transportation)

Example: DOT X5AB 2323

The last four digits indicate:

Week of manufacture
Year of manufacture

So: 2323 = 23rd week of 2023

Tires generally should not be used beyond 6–10 years from production.

UTQG Rating (Uniform Tire Quality Grading)

Includes three components:

Treadwear

Indicates expected longevity (e.g., 400, 500, 700).

Traction

Grades: AA, A, B, C

Temperature

Indicates heat resistance (A, B, C)

Metric vs. Flotation Tire Sizes

Most passenger vehicles use metric sizing.

Example: 225/65R17

However, off-road vehicles may use flotation sizing.

Example: 33x12.50R15

Breakdown:

33 = Overall tire diameter in inches
12.50 = Tire width in inches
15 = Rim size

These are common on lifted trucks and off-road SUVs.

Must Read: All Season Vs All Weather Vs Winter Tires - Complete Guide

How Tire Size Affects Vehicle Performance?

Changing tire size impacts:

Handling

Wider, lower-profile tires improve cornering response.

Ride Comfort

Higher aspect ratios absorb road imperfections better.

Speedometer Accuracy

Changing overall tire diameter alters speed readings.

Fuel Economy

Larger or wider tires increase rolling resistance.

Can You Change Tire Size?

Yes — but within limits.

Professionals recommend staying within: ±3% of original overall tire diameter.

Changing tire size may require:

Wheel alignment
Speedometer recalibration
Suspension clearance checks

Common Mistakes When Reading Tire Size

Ignoring load index
Confusing rim diameter units
Mixing different tire sizes on same axle
Choosing lower speed rating than recommended
Assuming wider is always better

Final Thoughts

Reading tire size is not just about understanding numbers — it’s about understanding how your vehicle connects to the road.

When you decode a marking like: P225/65R17 102H

You now understand:

The tire category
Width
Sidewall height
Construction type
Rim compatibility
Load capacity
Speed capability

That knowledge helps you:

Make informed buying decisions
Maintain vehicle safety
Optimize performance
Avoid costly mistakes

Not sure which tire size is right for your vehicle? Our experts at Johnston & Magwood Tires & Auto can help you choose the perfect tires for your car, SUV, or truck. From tire selection to installation and balancing, we ensure your vehicle stays safe and performs at its best.