Understanding Tire Sizes: Charts, Conversions and Recommendations

Understanding tire sizes can be very important when you're shopping for replacement tires for your vehicle. The safest thing to do is to replace the tires according to the size recommended by the original manufacturer specifications. However, there are a number of situations that may warrant a different tire size for your vehicle. If you plan to customize your vehicle in any way, larger-sized or low profile tires may be on your shopping list. In these cases, you'll want to make sure you understand how to read tire sizes, tire size conversion and which tires will fit properly based on the year, make, model and customizations of your vehicle.

How to Read Tire Sizes

Tire sizes are always listed in a format that can easily be decoded. There are many variables that can be included in this code, but the most common is displayed in a set of 3 numbers, such as P185/65R15. This code provides information about the dimensions of the tire, as well as intended purpose, load capacity and durability based upon speed and temperature changes. Following is an explanation of each part of the code P185/65/R15:

P - this is a tire of P-metric size, designed to fit on passenger vehicles. Other examples of this portion of the code are T for temporary spare, LT for light-truck metric, and if no letter is present, this indicates a Euro-metric tire.
185 - this is the section width from the inner to outer sidewall
65 - this is the aspect ratio, which indicates the tire's sidewall height from rim to tread
R - indicates that this is a radial tire. Other examples include D for diagonal and B for belted.
15 - indicates the tire and wheel diameter that must be matched together (ie: 15" rims require a 15 in this portion of the tire's code)

Tire Size Conversion

Tire sizes are normally listed in metric measurements. At times, it may be necessary to convert these dimensions into inches. There are some very simple calculations that can result in the measurements you require. Additionally, there are some quick and easy online tools that can make these calculations for you, as well as online diagrams and charts that list the final calculations already. If you wish to learn how to calculate tire conversions yourself, or the math behind the calculations, you can see the details worked out using the original example of P185/65/R15 as follows:

Divide section width by 25.4 - in the example: 185/25.4 = 7.28"
Multiply the aspect ratio with the result from above - in the example: 7.28 x 0.65 = 4.73"
Outside diameter is calculated using the last digits in the code. The section width is multiplied with aspect ratio, then multiplied by 2 (for the upper and lower sidewalls), then added to the outside diameter - in the example: 185mm x .65=120.25mm x 2=240.5mm + 381mm(15")= 621.5mm or 24.47"