Tire Sizes: Descriptions, Conversions, and Chart
Standard Inch Size
The breakdown of a tire’s size in standard inch measurements is straightforward. We will look at a common tire size for many stock SUV’s and Trucks:
Size: 31×10.50r15
The size of a tire is split into three components. First comes the overall diameter (in this case, 31 inches), followed by the tire’s tread width (10.5 inches) and finally the diameter of rim it is made for (15 inches).
Metric Size
Metric size tires are somewhat more confusing, because the tire’s size is actually a mixture of metric and inch units. But most of us are familiar with this.
Size: 265x75r16
In the “modified metric system” or “U.S. metric system” as it might be called, the width of the tire’s tread is called out first, in this case 265 mm (milli-meters). The second number describes the height of the tire’s sidewall, but it is described as a percentage of the width of the tire. So, in our case, the tire’s sidewall is 75% the width of the tire, or 199 mm. Finally, the third number is the diameter rim the tire is designed for, but because the U.S. uses inch-size rims the diameter is described in inches (in other parts of the world, this number may instead be described in millimeters). In our case, the tire is made for a 16 inch rim.
Converting Standard Inch to Metric Size
It can be useful to know how to convert from inch size to metric, in order to know what a certain size tire would correspond to in the metric sizing standard. We will back-convert the following tire size from the above metric to inch conversion:
Inch Size: 31×10.50r16
First, we already know the width of the tire (10.5 inches) so we can convert that number to millimeters by multiplying by the number of millimeters in an inch (25.4 millimeters per inch):
Tire Width: 10.5 inches X 25.4 mm per inch = 266.7 mm
Metric sizes are most often quoted in even divisions of 5 mm, so we can round the above result down to 265 mm.
Next, we need to detemine the sidewall height. To find the sidewall height, we take the tire’s overall diameter (31 inches), subtract the diameter of the rim (16 inches), and divide that number by 2:
Sidewall Height: ( 31 inches - 16 inches ) / 2 = 7.5 inches
Now that we know the sidewall height, we can find the percentage value used in the metric sizing standard by dividing the sidewall height (7.5 inches) by the tread width (10.5 inches):
Sidewall Percentage: 7.5 inches / 10.5 inches = 0.71 = 71%
Like the tire width, the tire’s sidewall percentage is quotes in units of 5, so our tire sidewall percentage is somewhere between 70% and 75%.
So, based on our calculations, a 31×10.50r16 tire is approximately equal in size to a 265x75r16 tire.
Pre-Calculated Tire Sizes (for your convenience)
