"P" and "LT"

The "P" in the beginning of the number series tells you that it is either a "performance" or "passenger" type of tire. If there is an "LT," then it is a "light-truck tire."

Tire Width

The next three numbers right before the forward slash designates the tire's width. This measurement is given in millimeters.

Aspect Ratio Number

The next set of numbers after the forward slash give the "aspect ratio." This is basically how tall it is. The number is found by dividing the tire's height by the width. The better performing tires will have a lower aspect ratio number.

"R"

If you see an "R," it means radial, which shows how the tire was made, or its "ply composition." Ply composition is how many layers of fabric coated with rubber are in the tire. The more layers, the more weight the tire can support. This is shown by the number after the "R," but some tires may not have this shown.

Speed Rating

If there is a letter after the weight number, this designates the speed rating. The speeds shown are in the area of 99 to 186 miles per hour. The speed rating is based on how fast a car can be driven for long time periods.

Source:

Walmart: How do I Find the Right Sized Tires?

Dunlop Tires: How to Read a Sidewall

U.S. Department of Transportation: Tire Safety: Everything Rides on It

More Information:

U.S. Department of Transportation: Tire Safety