THE SIZE AND MARKINGS

LEARNING OBJECTIVES

Learn how to read and understand the alphanumeric codes found on a tire’s sidewall, and why this information is crucial for safety, performance, and proper tire selection.

By the end of this lesson, you should be able to:

Understand the meaning of each part of a tire size code (e.g., P225/70R16 91S) and explain how it relates to tire dimensions, construction, load capacity, and speed rating.
Appreciate the importance of tire markings in selecting appropriate tires and ensuring vehicle safety, performance, and compliance.
Demonstrate the ability to locate and interpret tire markings on an actual tire and match them to a vehicle’s recommended specifications.

If you look at the sidewall of a tire, you’ll notice a string of letters and numbers like 205/55R16 91V. This alphanumeric code might look confusing at first, but once you understand it, you’ll realize it provides everything you need to know about that tire’s dimensions and performance limits.

Understanding tire markings is essential for ensuring safety, maintaining performance, and selecting the correct replacement tires. (Bridgestone Americas, n.d.)

DECODING THE MARKINGS:

Example:

P225/70R16 91S

P – VEHICLE TYPE

The first letter in the code refers to the intended use or classification of the tire.

P stands for Vehicle Type This means the tire is made for regular vehicles like sedans, hatchbacks, small SUVs, and minivans.

Other common letters:

LT = Light Truck: Designed for heavier loads or off-road use.
T = Temporary: A space-saver or “donut” spare tire.
ST = Special Trailer: For trailers only, not suitable for passenger vehicles.

225 – TIRE WIDTH

Next comes a three-digit number that represents the width of the tire.

In this case, 225 means the tire is 225 millimeters wide from one sidewall to the other.

A wider tire can improve traction and handling, but it also needs to match the vehicle’s specifications for safe and efficient driving.

70 – ASPECT RATIO

This two-digit number tells you the sidewall height as a percentage of the tire's width.

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

To calculate the sidewall height:

70% of 225 mm = 157.5 mm

This ratio affects ride comfort and tire flexibility. Higher aspect ratios (like 70) generally give a smoother ride, while lower ratios (like 45 or 50) offer more responsive handling.

R – CONSTRUCTION TYPE

The letter R refers to how the tire is constructed internally.

R means the tire uses Radial construction, which is the standard for modern tires.

In radial construction, the internal cords are arranged at 90-degree angles to the direction of rotation. This structure:
- Improves strength
- Enhances durability
- Provides better contact with the road

Nearly all passenger and light truck tires today use radial construction, which is why you'll commonly see this letter in tire sizes.

16 – RIM DIAMETER

The next number in the code tells us the wheel size the tire is meant to fit.

16 means the tire is made for a 16-inch rim (the diameter of the wheel it fits onto).

This measurement must match your vehicle’s wheel size exactly. A mismatch could lead to mounting issues, poor handling, or even tire failure.

91S – LOAD INDEX AND SPEED RATING

This final portion provides two key performance limits of the tire:

91 – Load Index

The number 91 corresponds to a load capacity of 615 kg (1,356 lbs) per tire.
This tells you how much weight each tire can safely carry when properly inflated.

S – Speed Rating

The letter S means the tire is rated for a maximum speed of 180 km/h (112 mph) under optimal conditions.

Other common speed ratings include:

T = 190 km/h
H = 210 km/h
V = 240 km/h

Choosing the correct speed rating ensures that the tire can handle the speeds your vehicle is capable of, especially during highway driving.

Why This Matters

Choosing the correct tire size and rating is critical for:

Safety – Preventing blowouts or tire failure due to overloading or overspeeding.
Handling & Performance – Ensuring proper traction, braking, and cornering.
Efficiency – Matching tire size to your vehicle ensures optimal fuel consumption.
Compliance – Automakers recommend specific sizes; deviating can affect warranties or legal inspections.

Tip: Always check the vehicle’s tire information label (usually found on the driver’s door frame) or the owner’s manual for recommended tire sizes.

SUMMARY

By learning to read tire size markings like P225/70R16 91S, you're gaining valuable knowledge that improves vehicle safety, ensures proper fit, and helps you make informed choices as a driver or technician. Understanding tire codes is more than just reading numbers it’s knowing your vehicle better and ensuring it runs safely and efficiently on the road.

In this lesson, you explored how to interpret each part of a tire size code, such as width, aspect ratio, construction type, rim diameter, load index, and speed rating. You learned how markings like "P225/70R16 91S" are used to describe a tire's specifications, and why this information is vital for selecting the correct tire for your vehicle. Tire markings aren't just technical data they’re a guide to safety, performance, and reliability on the road.

Ever wondered what all those numbers and letters on your tire mean? Learn how to read tire sizes and sidewall markings in this helpful video!

Decode the Sidewall!

Think you can read a tire like a pro? Put your skills to the test in this quick quiz on tire sizes and sidewall markings!

LIST OF SOURCES USED IN THE THIS WEBPAGE

Bridgestone Americas. (n.d.). Determining tire size. https://www.bridgestoneamericas.com/en/company/safety/choosing-tires/determining-tire-size

AutoWeb. (2013, June 11). How to read a tire size & understanding a tire sidewall - ABTL Auto Extras [Video]. YouTube. https://www.youtube.com/watch?v=HDQ40hjWrRM

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