How to read tire size: Foolproof guide for busy mechanics

Have you ever noticed the letters and numbers on a tire’s sidewall while changing tires and wondered what they mean? These markings show the make, model, size, and other details about your car’s tires. Knowing what they mean helps you choose the right tire for your customer, whether they want better performance or a unique look. This guide will help you understand how to read tire sizes and pick the best ones for your customers.

Where to find the tire sizes

Location	What you’ll find	Reliability
Driver’s door jamb sticker	OEM tire size and recommended inflation pressure	Best source; reflects the manufacturer’s specification
Tire sidewall	Current tire size, e.g. 225/65R17	Confirms what is installed, but may not be original if tires were changed
Owner’s manual	Factory tire/wheel specifications and pressure	Reliable; often in the glove compartment
Online tire finder	Size by year, make, model, and trim/option	Convenient, but confirm against the door placard

Even if you find the information, you cannot understand it unless you know how to decipher it. Yes, I know it feels like solving a puzzle. But it’s not that complicated once you know the basics, so here’s how to decode the information.

How to read tire sizes

Tire type

Different vehicles need different types of tires. The manufacturer-recommended tire type is often indicated by a letter at the beginning of the tire size sequence.

On cars, minivans, SUVs, and light-duty pickup trucks, you may find the letter P at the beginning of the sequence. P stands for P-metric, which indicates that the tire is designed primarily for passenger vehicles.

If at the beginning of the sequence, you see LT, it means the tire is designed for light trucks that carry heavy cargo or tow larger trailers. These types of tires are built to handle the additional demands of these applications.

If there are no letters at the beginning of the sequence, as shown in the picture above, the tire is usually a Euro-metric tire. These tires use a European sizing system and are commonly used on passenger cars and SUVs.

Remember, different tire types have different load limits and inflation needs. Picking a tire that matches the manufacturer’s specs helps your customer drive safely.

If you see T at the beginning of the sequence, it indicates a temporary-use tire, commonly used as a spare.

Tire width

This 3-digit number that appears at the beginning of the sequence specifies the width of the tire from sidewall to sidewall. This is measured in millimeters, and this helps determine the best tires for your car. Understanding the tire width is important because it decides the stability, mileage, braking, and handling of your car.

If the sequence begins thus, ‘P215’, then it means that this tire is for a passenger vehicle and has a width of 215 millimeters.

Aspect ratio

The three-digit tire width ends in a slash, closely followed by a two-digit number. This number shows the ratio of the tire’s height to its width. The bigger the aspect ratio, the bigger the tire’s sidewall will be. If you are wondering why this is important in a tire, it is because this ratio affects a vehicle’s performance, safety, and ride comfort.

A higher aspect ratio is equal to a taller sidewall. A taller sidewall provides more cushioning, and it absorbs road imperfections, ensuring a smoother ride. In a series, ‘P215/55’ indicates that the tire height is 55 % of its width.

Construction

After the aspect ratio, you’ll see a letter that shows the tire’s internal construction and stability. Most tires in the United States have an ‘R’ for Radial, which has been the standard for over 40 years. This means the cords inside the tire are arranged at 90 degrees to the direction of travel.

Wheel diameter

The following two digits show the wheel size, measured across the wheel. For example, P215/55 R 17 fits a 17-inch wheel. Knowing the wheel diameter helps you choose a tire that fits your car’s rim.

Load index

Next is the load index, which shows the maximum weight the tire can support when fully inflated. The number refers to a specific load capacity in a chart, ranging from 99 to 7385 lbs for numbers 1 to 150.

Heavier vehicles like SUVs need a higher load index than sedans. For example, a load index of 94 means the tire can carry up to 1,477 lbs.

Speed rating

The last letter in the sequence shows the tire’s maximum speed. Speed ratings usually match the car’s top speed. Just like the load index matches a weight, the speed rating letter matches a speed.

For example, a tire with a “W” speed rating is good for up to 168 mph, while “R” is up to 106 mph. Here are some other common speed ratings:

• Q – 99mph
• R – 106mph
• S – 112mph
• T – 118mph
• U – 124mph
• H – 130mph
• V – 149mph
• W – 186mph
• ZR – 186+

More infomation

Though this is the primary information most people look for in a tire, there is other significant data included in the tire, too. They include standard certifications and the year and place of manufacture. Some of the other data you can see while looking to read tire size are:

• DOT- indicates that the tire complies with all Federal Motor Vehicle Standards
• Tire Identification Number- includes factory location, week, and year of manufacture.
• Uniform Tire Quality Grading- indicates the traction, treadwear, and temperature capabilities of the tire.

Summing up

Knowing how to read tire sizes is super important for a mechanic, whether you’re a novice or a veteran. Your understanding of the jumble of numbers and letters on a tire’s sidewall is what makes sure you get the right tires for each vehicle. The right tires make a big difference in performance, handling, and safety. So, when you check a customer’s tires, take a moment to read these markings and make sure any replacements match the manufacturer’s specs. Happy wrenching!