Breaking the code: How to optimize tire size and performance specifications by reading your sidewalls

Tire sizes can be incredibly important when you are shopping for after-market wheels or specialty tires. The wrong-sized tire can cause performance issues and possibly damage to your car. Often, this detail gets overlooked when car owners are out shopping for mud tires, winter tires or fancy new rims. Your maker or dealer may offer a neumático size chart to help take the guesswork out of shopping, but if they don't, then you will need to de-code your tires.

Every tire has a series of letters and numbers stamped into the sidewall, and it's not for decoration. These letters and numbers give a great deal of detail about the vehicle, including where it was made. The numbers that consider tire sizing, though, are found in the first section, usually starting with the letter P (for passenger) or the letters LT (for light truck).

Section width and aspect ratio are the first two numbers that appear in the code. They are typically both measured in millimeters now, although earlier American cars were measured in inches. If your car is older, you may need to look online for a tire size converter to help calculate these measurements in millimeters. The section width is basically a measurement of the tread, from sidewall to sidewall. The aspect ratio is a measurement that reflects the ratio of the sidewall to the section width. So an aspect ratio of 70 means that the sidewall is 70 percent of the section width.

Wheel diameter

After the section width and aspect ratio, you will see a letter designating the type of construction (most tires are radial, so the letter you would most commonly see is R). Following this, you will see a number for the wheel diameter, which is represented in inches. If you have metric tires on your car (common in Europe), you will have to do a tire size conversion to get a number in inches.

Load index and speed rating

You will then see a number (the load index) followed by a letter (the speed rating, usually a T or H). These don't actually represent tire sizes, but are still very important numbers to consider when you are purchasing tires. Sizes may tell you what will fit on your current assembly, but the load index and speed rating will tell you a lot about how the tires will perform. The load index tells you the maximum amount of weight your tires will be able to carry safely, and the speed rating is attained by meeting government minimum standards for reaching and maintaining a given speed. Generally, the higher the speed rating, the better your vehicle will handle.

How to use these numbers

If you are just looking to replace some worn-out tires, then all you have to do is match the numbers to ensure you are getting the same thing you started with. However, you may want to make some changes if you are purchasing tires for different rims or weather situations. Whether you are making personal modifications or just looking for fresh tires, Sears can help find treads to help you optimize your vehicle's handling and performance.

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