Finding The Right Tire Size For Your Vehicle

Determing What Size Tires You Need

The correct tire for your vehicle will be a unique combination of different variables including the width, height, and diameter of the tire, as well as a combination the tire's load capacity rating and speed ability. To find out what size tires you need and what speed/load rating your vehicle requires, check your owner's manual. If the information is not listed there, it can usually be found inside the door jamb of the driver's door, inside the glove box door, or inside the fuel tank hatch.

Door Jamb Vehicle Placard

If you can't find the information in any of these locations, read the information off of the sidewall on your existing tires. Here is an example of the types of numbers you want to look for:

reading-tire-sidewall-info

You can write all these numbers down and then bring them to your local shop to see what tires are available in this size. In the above example, you could tell your local tire technician that you are looking for "185/75/14" tires with a "82S" speed and load rating. That's all the information you'll need to find tires that will fit your car as the manufacturer intended.

What Do The Numbers on The Sidewall Actually Mean?

Aspect ratio: The aspect ratio is essentially a measure of how tall your tire is. The larger the number, the taller the tire. The number is calculated by dividing the height of the tire by it's width, then turning the result into a percentage. Your typical all-season passenger car tire will have an aspect ratio somewhere between 65 and 80.
Load index: Your tire's load index lets you know how much weight the tire was designed to carry safely. By using the load index chart displayed below, you can match the rating up with the number of pounds the tire can support when it is inflated to its maximum air pressure. For example, if your load rating is 81, you can then determine that the tire was engineered to carry 1,019 pounds when fully inflated. Multiply this weight by four to find out the maximum carrying capacity for your vehicle. When buying aftermarket or replacement tires, it's important to find a model that matches the load index that was on the original equipment.
Speed rating: The speed rating is a number issued by the U.S. government which signifies how well a tire can reach and maintain a certain speed. Speed ratings are simply letters which correspond to a specific top speed. Keep in mind that these are not recommended speeds to drive at, but simply a way to get a better idea of how well the tire will handle (tires with a hire speed rating are likely to provide better traction and handling). When buying new tires, always choose 4 tires with identical speed ratings. Mixing up the specifications on your tires will result in serious and unsafe driving conditions. It's also not recommended that you downgrade the speed rating on your tires. Always follow the manufacturer's guidelines and buy tires with ratings that are identical to the original stock equipment.

Once you have all the information from your tire sidewall or vehicle placard, you can now narrow down your search for new tires. Other variables like treadwear, traction, and temperature grades may affect the type of tires you need, but for everyday purposes the size, speed, and load numbers are all you'll need. Contact your local tire shop for more information about the different types of tires. They'll assist you in finding the tires you want at the sizes you need.