How to Calculate Tire Size

If you’ve been thinking about getting some new tires for your SUV for better performance or your old tires have simply worn out, you have a lot of decisions to make. There are many different features you’ll have to choose from when shopping for tires, but the correct size is perhaps the most important thing to look for. Knowing what size you need, you can narrow down your search and find a set of tires that you know will fit and won’t give you any trouble during the installation process. Being confident in how to calculate tire size will help you make the best decision and avoid any issues when the time comes for some new rubber.

Why Tire Size is Important

When shopping for new tires, size is an essential consideration. The main reason why tire size is important is for safety and handling. With improperly sized tires, you could end up with poor traction, especially in wet or icy conditions, and if you’re buying tires for their grip, that can be a major issue. Tire size also severely affects the rolling resistance of your vehicle, which can have a big impact on your fuel efficiency. If your new tires are too big, for example, the engine will have a harder time producing the same amount of speed and you will end up using much more fuel than you otherwise would. Aside from the obvious issues of fitting the tires onto the rims and in the wheel wells, these performance aspects are reason enough to make sure you get the correct size.

What Tire Size Numbers Mean

When looking at your vehicle’s tires, you’ve probably noticed a string of letters and numbers on the sidewall. This code is a combination of all the different dimensions of the tire and will look something like this: P215/65R15 95H. By knowing what this code means, you can calculate the diameter of the tires and find the perfect set of replacement tires for your vehicle.

• Tire type – The first letter indicates the type of vehicle that the tire is meant for. Most of the time, it will be a P, which means the tire is for a passenger vehicle and meant for comfort. It may also say LT or ST, which is for a light truck or a trailer, respectively, and the tread will be much more focused on traction and load-bearing.
• Tire width – The numbers after the first letter represent the width of the tire in millimeters. This is the measurement from sidewall to sidewall and shows how much space the tire will take up from the outside edge to the inside of the wheel well.
• Aspect ratio – The aspect ratio is the number after the slash. It represents the height of the sidewall as a percentage of the width. This helps determine the size of the rim that the tire can be put on and helps ensure fitment.
• Construction Type – After the aspect ratio, there is another letter, which indicates the construction type of the tire. Most of the time, this will be an R, which stands for radial but you may also see B for belted bias or D for diagonal.
• Rim Diameter – The last two digits after the construction type indicate the rim diameter. This is an incredibly important part of the tire code and will help ensure that your new tires will fit on the wheels you have.
• Load Index and Speed Rating – The very last set of numbers and letters is the load index and the speed rating. It is very important that you match these numbers with your new tires since they indicate the kind of weight and speed they are designed to withstand.

How to Calculate Overall Tire Diameter

To calculate the overall tire diameter, which you will need in order to find the right tire replacements, you will need to do some math. If, for example, your tires have a code on them that is 215/55R17, you will be using these numbers to calculate the tire size. Using this example, you will multiply the tire width and the decimal of the aspect ratio, which is 215 x 0.55. This will give you the sidewall height, which is 118.25 millimeters. Then, you will convert this to inches by multiplying it by 25.44. This gives you 4.65 inches. Once you have this, you will double it and add the wheel diameter. In this case, the formula is (4.65 x 2) + 17. This results in your final overall tire diameter of 26.3 inches. With this figure, you can find the perfect tire size for your vehicle. No matter what size tires you’re looking for, ExtremeTerrain has you covered. With a wide range of sizes and construction types from the brands you trust, you can get your new tires and head out on the road in no time.