How to Read a Tire Size: Numbers & Letters Explained

If you have ever looked at the side of a tire and seen a code such as 225/65R17 102H, the numbers and letters can seem complicated at first. Each part of that code describes an important tire specification, including width, sidewall height, wheel diameter, load capacity, and speed rating.

This guide explains how to read a tire size step by step, what the tire numbers and letters mean, and which measurements matter when checking tire and vehicle fitment.

How Tire Size Numbers Work

Most passenger-vehicle tire sizes use a format similar to:

225 / 65 R 17 102 H

Although these values appear together on the sidewall, they describe different characteristics of the tire. Understanding each part makes it easier to compare tires, check specifications, and understand how a tire may affect vehicle fitment.

1. Tire Width: 225

The first number is the tire’s nominal section width in millimeters. In a 225/65R17 tire, the nominal width is 225 mm.

This does not mean every tire labeled 225 will measure exactly 225 mm when mounted. Actual dimensions can vary somewhat by tire model, manufacturer, measuring rim width, inflation pressure, and load.

A larger first number generally indicates a wider tire. For example, a 245-width tire is nominally wider than a 225-width tire, but width alone does not determine whether a tire will fit a vehicle.

2. Aspect Ratio: 65

The second number is the aspect ratio. It describes the sidewall height as a percentage of the tire’s nominal width.

For a 225/65R17 tire, the aspect ratio is 65. That means the calculated sidewall height is approximately 65% of 225 mm.

The calculation is:

225 × 0.65 = 146.25 mm

So the calculated sidewall height is about 146 mm.

A lower aspect ratio usually produces a shorter sidewall relative to tire width, while a higher aspect ratio produces a taller sidewall.

3. What Does R Mean on a Tire?

The letter R in a tire size stands for radial construction. Radial tires use body-cord plies arranged generally across the tire from bead to bead.

Radial construction is common on modern passenger vehicles, SUVs, and light trucks. The R does not mean rim or radius.

4. Wheel Diameter: 17

The number after the construction letter identifies the tire’s intended wheel diameter in inches.

A 225/65R17 tire is designed for a 17-inch wheel. A tire marked R18 is designed for an 18-inch wheel, while an R20 tire requires a 20-inch wheel.

This dimension must match the wheel diameter. A 17-inch tire cannot simply be installed on an 18-inch wheel.

What Do the Numbers After the Tire Size Mean?

A complete tire description often contains additional numbers and letters after the basic size. For example:

225/65R17 102H

The final 102H is often called the tire’s service description.

Load Index: 102

The load index is a numerical code associated with the maximum load a properly inflated tire can support under specified conditions.

It is important not to replace a tire with one that has an inadequate load capacity for the vehicle. Check the vehicle placard, owner’s documentation, and tire manufacturer’s specifications when selecting a replacement.

Speed Rating: H

The final letter is commonly the tire’s speed rating. It identifies a standardized speed capability under specified test conditions.

Common speed-rating letters include T, H, V, W, and Y, but the correct replacement specification depends on the vehicle and tire application.

A speed rating should not be interpreted as a recommendation to drive at that speed. Vehicle condition, road conditions, tire pressure, load, temperature, local laws, and many other factors affect safe operation.

What Do P and LT Mean on a Tire?

Some tire sizes include letters before the width number.

• P generally identifies a P-metric passenger tire.
• LT generally identifies a tire intended for light-truck applications.

For example, you may see P225/65R17 or an LT-designated tire size. These designations can be associated with different load and inflation standards, so a P-metric tire and an LT tire should not be treated as automatically interchangeable simply because other size numbers look similar.

Tire Size Example Breakdown

Marking	Example	What It Means
Width	225	Nominal section width in millimeters
Aspect ratio	65	Sidewall height is 65% of nominal width
Construction	R	Radial construction
Wheel diameter	17	Designed for a 17-inch wheel
Load index	102	Standardized load-capacity code
Speed rating	H	Standardized speed-rating code

Does Tire Size Tell You the Actual Tire Diameter?

Not directly, but the basic metric tire size contains enough information to calculate an approximate overall diameter.

The calculation uses:

• tire width
• aspect ratio
• wheel diameter

For example, the sidewall of a 225/65R17 tire is approximately 146.25 mm tall. Two sidewalls plus the 17-inch wheel diameter give an approximate unloaded tire diameter.

For quick calculations and conversions, use the Tire Size Converter or browse our tire size dimensions.

Why Two Tires With the Same Size Can Measure Differently

A tire size such as 225/65R17 is a standardized nominal designation, but two different tire models carrying that size do not necessarily have perfectly identical mounted dimensions.

Actual measurements can vary because of:

• tire manufacturer and model
• tread design and tread depth
• approved rim-width range
• measuring rim width
• inflation pressure
• vehicle load
• manufacturing tolerances

For clearance-sensitive applications, always check the manufacturer’s published specifications for the exact tire model rather than relying only on calculated dimensions.

How to Find the Correct Tire Size for Your Vehicle

The tire currently installed on the vehicle can provide useful information, but it should not always be assumed to be the original or correct specification. A previous owner may have changed the tire or wheel size.

Useful places to confirm vehicle tire specifications include:

• the tire and loading information placard
• vehicle owner’s documentation
• manufacturer fitment information
• reliable vehicle-specific tire and wheel data

You can also use our Vehicle Tire & Wheel Fitment section to research available fitment records by year, make, model, and trim as the FitmentCompass database expands.

Can You Use a Different Tire Size?

Sometimes a vehicle can use more than one tire size, but a different size should not be selected based only on whether it physically mounts to the wheel.

Changing tire size can affect:

• overall tire diameter
• ride height
• speedometer and odometer readings
• fender and suspension clearance
• tire-to-wheel compatibility
• load capacity
• handling
• brake and drivetrain behavior on some vehicles

Before changing sizes, use the Tire Size Comparison Calculator to compare width, sidewall height, diameter, circumference, revolutions per mile, and estimated speedometer changes.

How to Read a Tire Size at a Glance

Using 225/65R17 102H as an example:

• 225 tells you the nominal tire width in millimeters.
• 65 tells you the sidewall height is approximately 65% of the tire width.
• R identifies radial construction.
• 17 identifies the wheel diameter in inches.
• 102 is the load index.
• H is the speed rating.

These markings are an important starting point, but correct fitment also depends on the exact vehicle, wheel specifications, load requirements, clearances, and manufacturer recommendations.

Frequently Asked Questions

---

Fitment note: Tire dimensions calculated from a size code are theoretical values. Actual mounted dimensions may differ by tire model, manufacturer, approved rim width, inflation pressure, and load. Always confirm the exact vehicle and tire specifications before purchasing or installing a replacement size.