Tire Basics

9 min read

One of the most essential parts of your vehicle is the tires. These incredible pieces of technology are the only part of your vehicle that connects with the road and are highly engineered to support the entire weight of your car. The surface of the tire that touches the road is roughly the size of your palm and is called the contact patch. This contact patch is responsible for keeping you firmly planted on the road.

Because of the critical aspect tires play in our driving experience, you must understand the ins and outs of tires to ensure you buy the correct tires for your vehicle, accurately maintain your tires, and can handle every adventure going forward.

Learn everything you need to know about tires and how they impact your driving experience from BFGoodrich. How tires are made, tire speed rating, how to read a tire sidewall...we have it all!

Tire Anatomy & Tire Construction

There's a lot more to a tire than just rubber on a rim. The typical radial tire consists of six main parts.

Inner Liner

The days of tubed tires are over. Today's modern tires utilize synthetic rubber instead - specifically butyl rubber* - which is virtually impenetrable to water and air. However, over time, there will be slight air loss, so check your pressure every month.

Crown Plies (or Belts)

Crown plies provide the rigid base for the tread.


The tread provides traction and cornering grip for the tire and is designed to resist wear, abrasion and heat.

Carcass Ply

The carcass ply is the layer above the inner liner and consists of thin textile fiber cords (or cables) bonded into the rubber. These cables are largely responsible for determining the strength of the tire.


The beads clamp firmly against the tire's rim to ensure an airtight fit and keep the tire properly seated on the rim.


The sidewall protects the side of the tire from impacts with curbs and the road. This is where the important details about the tire can be found, such as tire width and speed rating.

Tire Speed Rating & Tread life

A through Z. Find out what rating offer longer mileage, better grip, and a smoother ride. The most common tire speed ratings, speeds and vehicle usage are as follows:

Tire Speed Ratings

The Origin of Speed Ratings

We can thank Germany's famous Autobahn for tire speed ratings. Tire speed ratings range from A (the lowest) to Y (the highest). But the chart is not completely in alphabetical order. For example, H is between U and V, with the common perception that H stood for "high performance" at one time. As manufacturers continue to add speed to their vehicles, tire speed ratings evolve to match the speeds. For example, Z was the highest rated speed at 149+ until W & Y were used to match the higher speeds of exotic sports cars.

Speed ratings refer to more than just speed

Contrary to the name, speed ratings aren’t just about speed. They’re also about ride comfort, wear, and cornering ability. Typically, the higher the speed rating, the better the grip and stopping power, but the lower the tread life. You can always increase the speed rating of the tires on your vehicle for improved performance but can never decrease it without reducing the vehicle top speed to that of the lower speed rating selected.

Mixed Speed Ratings

We certainly don’t recommend it, but if tires of different speed ratings are mounted on a vehicle, the lower speed-rated tires should be placed on the front axle regardless of which axle is driven. This is to prevent a potential oversteer condition. Vehicle handling may be affected, and the vehicle’s speed capacity is now limited to the lowest speed-rated tire. For best performance, it is recommended that the same size and type of tire be used on all four wheel positions.

Load Index

Load Index

235/55R17 99H

The load index (99) is the tire size’s assigned numerical value used to compare relative load carrying capabilities. The higher the tire’s load index number, the greater its load carrying capacity.

97 = 1,609 pounds
98 = 1,653 pounds
99 = 1,709 pounds

A tire with the higher load index than that of the Original Equipment tire indicates an increase in load capacity. A tire with a load index equal to that of the Original Equipment tire indicates an equivalent load capacity. A tire with a lower load index than the Original Equipment tire indicates the tire does not equal the load capacity of the original.

Typically, the load indexes of the tires used on passenger cars and light trucks range from 70 to 110.

Reading a Tire Sidewall

The numbers and letter on the side of your tire have very specific meanings. A tire's sidewall is pretty much what it sounds like - the outer and inner "walls" on the sides of a tire, if facing a tire on its side. Every sidewall has its own unique information that is divided into four main sections:

Tire Specs

Tire Type

This designates the type of vehicle the tire fits. P is for passenger metric. Other letters are LT (for light truck), T (for temporary spare) and ST (for special trailers). If your tire has no letter, it signifies that your tire is a euro "metric" size.

Tire Width

Also called Section Width, this is the width of the tire (or thickness) in millimeters, if measured from a tire's widest point of its outer sidewall to the widest point of its inner sidewall. Why millimeters? It originated in Europe, which uses the metric system.


This identifies the tire's aspect ratio, which is the relationship of the tire's sidewall height to the tire's width. In this example, the sidewall height of the tire is 55% of its width. The lower the ratio, the smaller the sidewall height, which means better cornering, but a rougher ride.


This is the tire's internal construction, which is "radial". Almost every tire on the road has radial construction, which means the cords of the carcass plies inside the tire "radiate" directly across from one side of the tire to the other. Other letters used are D, for diagonal construction, and B, for belted.

Wheel Diameter

This number (in inches) indicates that the tire is designed to fit on a wheel with an 18-in diameter.

Load Index

This indicates how much weight the tire is certified to carry at maximum safe inflation. It doesn't mean 97 pounds, because it's actually an assigned value that corresponds with its "actual" load capacity found on a load index chart. If you look up 97 on the chart, you'll find 1,609 pounds.

Speed Rating

This indicates the maximum safe speed at which a tire is certified to carry a load under specified conditions. Speed ratings range from A (lowest) to Y (highest), with one exception: H falls between U and V. To find the maximum speed for your tire, refer to the speed rating chart*. Exceeding the lawful speed limit is neither recommended nor endorsed.

Department of transportation safety code

This assures that your tire complies with all Department of Transportation (DOT) safety standards. After the DOT insignia is your tire's identification number, which begins with the tire's manufacturer and plant code where the tire was manufactured (two numbers or letters). The ninth and tenth characters tell the week the tire was manufactured. The final number(s) signifies the year the tire was manufactured.


Department of Transportation Safety Code


Tire's manufacturer and plant code


Denotes tire size


Optional characters that identify brand and other significant characteristics of the tire


Denotes the week the tire was produced


Denotes the year the tire was manufactured

utqg code

The Uniform Tire Quality Grading (UTQG) was established by the National Highway Traffic Safety Administration (NHTSA) to test tires following government prescribed test methods and then grade each tire on three main components:


Treadwear: This is the wear rate of the tire, comparable only to other tires within a tire manufacturer's line. 100 is the baseline grade. Therefore, a tire with 200 would theoretically last twice as long on the government's course compared to a tire with 100.

Traction: Traction grades are AA, A, B and C (with AA being the highest grade). They represent the tire's ability to stop straight on wet pavement as measure on a specified government track. Any tire rater under C is considered unacceptable for road travel.

Temperature: The temperature grades, from highest to lowest, are A, B and C. These represent the tire's ability to dissipate heat under controlled indoor test conditions. Any tire rated below C is considered unacceptable.


Some tires have unique benefits, as showcased with specific icons. For example, a Mountain Snowflake symbol tell you that the tire meets or exceeds industry-established snow traction performance requirements.

The Right Tire for your vehicle

To find out what tires are right for your car, check your vehicle's OEM door sticker or your vehicle's owner's manual.

Tire Size

OEM stands for Original Equipment Manufacturer. Quite simply, these are the tire specs that originally came with your vehicle, according to the manufacturer (Honda, Ford, etc). It includes the tire size, rim size, the aspect ratio, etc.

Tire Size

Tire Pressure

The correct tire pressure to inflate your tires can be located on the vehicle or tire placard or Vehicle Owner's Manual and is the proper tire pressure to maintain, so your tires can give you the maximum performance, including gas mileage and tread life. For proper results, make sure your tires haven't been driven for three hours before you check for air pressure. (Friction creates heat, which increases tire pressure and gives improper readings.) Be aware that the air pressure displayed on the tire sidewall is the maximum pressure, not the recommended tire pressure.

Tire Pressure


Our Trip Planning section gives lots of helpful information to get you started.

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