Tire Size Conversion
Learning how to do tire size conversions is 
necessary due to the different systems being used. Besides the standard system, there is also the P-metric system.
The Standard and P-Metric Measuring Systems
The standard system describes the tire via the diameter, the rim / wheel size and its width. These figures are stated in inches. The metric system, on the other hand, defines the tire via 3 numbers (i.e., P265/65/R15). The first three numbers refer to the tire width in millimeters.
How to Convert from Standard to P-Metric
Step 1
Get a calculator and multiply. Get the tire width and multiply it by 25.4. For instance, if the tire diameter is 31.6 x 12/16 (12 inches in diameter), multiply 12 by 25.4. The answer is 304.8 mm.
Step 2
To continue the tire size conversion, subtract from the wheel (rim) size from the diameter of the tire. Divide the figure by two. This will give you the rim to tread distance. Using this example, the distance would be 7.8 inches (31.6 – 16) / 2 = 7.8).
Step 3
To get the aspect ratio, divide the distance from the rim to tread via the width of the tire (inches). Next, multiply the figure by 100. In this example, that would be 7.8 / 12 x 100 = 65.
How to Convert P-Metric to Standard
Step 1
Get a calculator to start the tire size conversion. Divide the tire width by 25.4. Using the P265/65/R15 figure, the result would be 10.43 inches (265 / 25.4 = 10.4).
Step 2
Multiply the aspect ratio and the width (this will be in inches). Divide the result by 100. This will give you the rim to tread distance. In this case that would be 6.78 niches (10.43 x 65/100 = 6.78).
Step 3
To determine the tire size diameter, multiply the rim to tread distance by two. Add the wheel dimensions to the result. In this example, that would be 28.56 inches 6.78 x 2 + 15 = 28.56.
The P-Metric System Explained
The P235/45/R17 94H will be the example here. The first letter indicates the service type. P means passenger car. If it’s a light truck, the letters would be LT. If there is no letter, it means it is a Europe metric tire.
The first three letters refer to the section width. This is measured by the sidewall’s widest section. This is always measured in millimeters. The proceeding two numbers refer to the aspect ratio. This is also measured in millimeters.
The tire construction type is denoted by the letter R. In most cases the R or radial ply is used. It is the standard type used in US light trucks and cars. The two numbers after the letter are the tire’s rim diameter. The final letter determines the speed rating and the load index.
For vehicle owners, learning how to do tire size conversions will definitely be an asset. Putting on the wrong tires can lead to problems, so knowing how to assess tire dimensions will be necessary.
