• P = passenger . If it said "LT" it would mean "light truck".
• 275 is the section width (in millimeters) from one sidewall to the other at the tire's widest point (the bulge) when mounted on a specifically sized rim and inflated to the recommended pressure. The section width varies approximately 0.2" for every 0.5" change in rim width.
• 60 is the "aspect ratio" and determines the sidewall height. On this example, the sidewall height is 60% of the section width (275 mm) and is 165 mm, again, based on the proper rim width and inflation pressure.
• R means Radial. I suppose there are designations for Bias Ply, but those tires are very rare.
• 15 is the rim diameter in inches.
• 107 The 107 represents the tire's load rating. As the number goes up, the load rating of the tire goes up. Each number (i.e. 107, or 93, or whatever) represents a certain weight that particular tire can handle in proper conditions.
• H The H is the speed rating for the tire and represents the maximum sustained speed the tire can withstand. There is a lot of fine print regarding that subject such as the load on the tire and the pressure inside the tire, so check on that before subjecting your tires to such "abuse".

Convert Old Labeling to New Labeling

• E78-14 converts to P195/75R14
• F78-14 converts to P205/75R14
• G78-14 converts to P215/75R14
• H78-14 converts to P225/75R14
• E78-15 converts to P195/75R15
• F78-15 converts to P205/75R15
• G78-15 converts to P215/75R15
• H78-15 converts to P225/75R15
• L78-15 converts to P235/75R15

Speed Ratings

• Q capable of speeds up to 100 mph.
• R capable of speeds up to 106 mph.
• S capable of speeds up to 112 mph.
• T capable of speeds up to 118 mph.
• U capable of speeds up to 124 mph.
• H capable of speeds up to 130 mph.
• V capable of speeds up to 149 mph.
• W capable of speeds up to 168 mph.
• Y capable of speeds up to 186 mph.
• Z capable of speeds above 149 mph.

Tire Height and Width Conversion

The way to figure out the width and height in inches from your P-metric tire designation is as follows: Take the section width, in this case 275 mm, and divide by 25.4 because there are 25.4 millimeters in an inch. Multiply that by the aspect ratio, in this case 60%, or .60. That will give you the height of one sidewall in inches. Multiply by two because you have two sidewalls from the ground to the top of the tire, and add the rim diameter. Presto. Piece of cake!

An example: The diameter of a P275/60HR15 tire is: 275mm / 25.4 x .60 x 2 + 15 = 27.99" diameter.

For specific tire diameters based on the p-metric designation, scroll down or click here.

Backspacing

Here's a graphic definition of "backspacing":