P225/55R16… What is that? Well, that is a passenger, radial tire that is 8.86 inches wide, has a sidewall that is 4.87 inches tall, a total diameter of 25.74 inches, a total circumference of 80.84 inches or 6.74 feet and takes 783 revolutions to go a mile.

That may sound like a lot of numbers, but keep reading and I will explain it all to you.

The example above is known as a P-Metric tire size. First, let’s start by understanding what each part means. The first letter P tells us that it is a tire intended for a passenger vehicle.

The first number is the width of the tire, from sidewall to sidewall, in millimeters (mm.)

The number after the / is called the aspect ratio. It represents the height of the sidewall as compared to the width of the tire. In the example, the tire has an aspect ratio of 55 (or 55% of the width of the tire,) which calculates to 225 * .55 or 123.75 mm (4.87 inches.)

The R represents that this is a radial tire. This could also be B for Bias or D for Diagonal, but those types of tire design are rarely used.

The number after R represents the wheel size in inches.

So, now that you know what each part of the P-Metric system means, we can plug the numbers into a few simple formulas to tell us even more.

1) Convert mm into inches.
mm / 25.4 = inches

2) Sidewall height in inches.
(Tire Width x Aspect ratio) / 25.4 = sidewall height in inches.

3) Diameter of the tire.
(Sidewall height(inches) x 2) + Wheel size = diameter in inches.

4) Circumference of the tire.
Diameter x 3.14 (pi) = circumference in inches.

5) Convert inches into feet.
inches / 12 = feet

6) Revolutions per mile.
5280 (1 mile) / circumference (in feet) = revolutions per mile.

7) RPM (Revolutions per minute.)
Speed (Mph) x 88 (converts to feet per minute) / Circumference (in feet) = RPM