Tire Sizes

 

Most people trained in science and engineering, learn to convert between various units of measure such as meters to feet, kilometers to miles (see prior post where one conversion has a link to the Fibonacci sequence), Fahrenheit to Celsius and countless others. One thing that bothers me is the use of mixed units. One example is automobile sizing.

Look at the sidewall of your tires and you’ll find a number like:

P 215 / 65 R 15 89 H

For this blog, I want to focus on just the three numbers that indicate the size of the tire (width, height, and rim diameter): 215, 65, 15.

These numbers are explained along with the other tire indicators:

•       P stands for the tire type (Passenger)
•       215 is the tire width in mm
•       65 is the aspect ratio (meaning the height of the tire as measured from the rim to the tread is 65% of width of the tire – in this case 65% of 215 mm or 140 mm)
•        R is the construction (Radial)
•        89 is the load index (indicates a maximum load of 1279 pounds)
•        H is the speed rating (indicates a maximum of 130 mph)

So, we have three numbers describing the dimensions of the tire, one in inches, one in millimeters and the final expressed as a percentage of the width. I believe European automobile tires use the same mixed-unit system (see this tyre size guide), but I will update this post if a reader corrects me.