Did you know that the United States is one of the only countries in the world that does not use the metric system as a national standard? That being said, we still see regular use of metric measurement intertwined in our everyday lives. For Americans who are used to American standard (inches, feet, yards), the metric conversion can take some effort to determine. And when it comes to tires, the metric size listings can be even more confusing. Conversions are required, and on top of that, the sizes aren’t listed in the same way as, say, an LT tire (with sizes shown in inches).
LT / Wide Base / Numeric / Flotation Sizes
Some larger and heavy-duty tires—and some earlier model tires—are categorized with an “LT” (Light Truck) designation, or also as “numeric,” “wide base,” or “flotation” sizes. These tires are listed showing the tire diameter (or height), then the tire width, [usually] followed by “R” to represent radial construction, and lastly the wheel/rim size. Some heavy-duty tires will not list the actual diameter but instead just the width and wheel size.
Let’s break down the following tire size: 35x12.50R15
35 is the tire diameter, or height, in inches.
12.50 is the width of the tire, in inches
15 is the wheel size, in inches
Metric & P-Metric Sizes
Most passenger car tires today are “metric” or “P-metric” (similar, with the “P” designating passenger car use). Metric tire sizes are listed with the width first, then the sidewall height (shown as a percentage of the tire width), [usually] followed by an “R” to designate radial construction, and lastly the wheel/rim size. This type of size listing can be especially confusing because it not only deals in inches and millimeters, it also gives you a percentage that you’ll need to use to do the math to find the tire diameter.
Let’s break down the following tire size: 225/55R17
225 is the tire width, in millimeters, from sidewall to sidewall
55 is the aspect ratio. This percentage compares the tire's section height with the tire's section width. For example, this 55 means that the tire's section height is 55% of the tire's section width.
17 is the wheel size, in inches