2. How do you read the sidewall of a tire?

On the side of your tire you'll find the size, make, and model of the tire, along with its maximum inflation and load, speed rating, and safety warnings. Sizing on today's vehicles usually looks like this:

P235/75R15 105S (cars) or
LT235/75R15 104/101Q (trucks)

The P means that the tire is designed for a Passenger car, LT for Light Trucks. If you choose to use a P rated tire on a vehicle that originally took an LT, the maximum load carrying capacity of the tire should be reduced by 9%.

The 235 is called the section width and is defined as the width of the tire in millimeters from sidewall to sidewall when the tire is fitted onto a rim of the recommended width. The width of the tread is slightly less than the section width.

The 75 is the Aspect Ratio, which specifies that the height of a tire's sidewall is 75 percent of its section width.

The R specifies that the tire is a radial tire. Most of today's tires are radials, but occasionally you may see a D or a B on the tire indicating that the tire is a bias ply tire. Never mix radial and bias ply tires on a vehicle.

The 15 is the diameter of the wheel in inches.

The 105 or 104/101 indicates the tire's load index as established by the Rubber Manufacturers Association. Make sure that the tire you replace has the same carrying capacity as the one you are taking off. You can do this by using a tire with the same load index, or by checking the maximum load capacity of the tire which is printed in fairly small print near the wheel. On trucks you often won't see this designation in which case you should look for a Load Range such as C, D, E, or F.

The S or Q is the speed rating of the tire, which indicates the tire's maximum speed.

Some common speed ratings are: Q = 99 MPH; S = 112 MPH; T = 118 MPH; U = 124 MPH; V = 149 MPH; and Z for cars that won't fall apart at over 149 MPH. The speed rating is placed just before the R on some older tires.

You can also consider this rating as a Performance Rating as tires with high speed ratings are built to withstand higher lateral and g-forces, resulting in improved handling even at slow speeds. If you want your Geo to handle like a BMW, upgrade the speed rating.

We do not recommend using a tire with a speed rating lower than that which the vehicle manufacturer recommends for your car, or mixing tires with different speed ratings on any vehicle.