• 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".

• 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

• 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.

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 is the distance from a wheel’s mounting surface to the farthest point inboard on the wheel. Make sure to measure from the exact point where the wheel will contact the car, not from any depression that may be between the lug nuts. Deeper backspacing moves the wheel and tire towards the center of the car; shallower moves them outward away from the center of the car.

For very clear information about backspacing, frontspacing, bolt circle, and general wheel clearance go to the Wheel Vintiques' Website.

Copyright © 2007 Bruce Johnson and Craig Watson