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Tire Information

Tire Terms |  What Tire Numbers Mean  |  Conversion of Old Size to New Metric Size
Speed Ratings  |  Calculating Tire Height/Width  |  Tire Diameters for 14" to 18" Wheels
A very good link to calculate tire sizes

Tire Terms

• M+S: A standard for all-season tires indicating traction capability including mud, snow, rain, and slush.
• Sidewall: The portion of the tire between the tread and the bead.
• Contact Patch: Also known as the footprint, this is the portion of the tread that is in contact with the driving surface.
• Bead: The portion of the tire that is retained by the rim, strengthened by a steel wire wrapped in tire plies.
• P-Metric: The most common currently-used system of tire sizing for passenger vehicle tires.
  
• LT-Metric: The most common currently-used system of tire sizing for light truck tires.
  
• Traction: A is best, C is "acceptable", and it represents a tire's ability to stop on wet pavement in a certain controlled situation.
  
• Temperature: A is best, C is "acceptable", and it represents a tire's resistance heat generated by friction between the tire and roadway.
  
• Tread Wear: 100 is the baseline number against which all tires are rated, the higher the number the better. A tire graded 300 should last three times as long as one graded 100 under exactly the same conditions. These ratings are NOT comparable between different tire manufacturers.

What The Numbers Mean

• 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 determines the sidewall height. On this example, the sidewall height is 60% of the section width (275 mm) or 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 accommodate.
• 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 Sizes to New Metric Size

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

Speed Ratings

• Q rated capable of speeds up to 100 mph.
• R rated capable of speeds up to 106 mph.
• S rated capable of speeds up to 112 mph.
• T rated capable of speeds up to 118 mph.
• U rated capable of speeds up to 124 mph.
• H rated capable of speeds up to 130 mph.
• V rated capable of speeds up to 149 mph.
• W rated capable of speeds up to 168 mph.
• Y rated capable of speeds up to 186 mph.
• Z rated 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 example we'll use 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!
Example: The diameter of a P275/60HR15 tire is: 275mm / 25.4 x .60 x 2 + 15 = 27.99" diameter.

Tire Diameters

Simply find the wheel diameter in which you're interested, then cross the section width with the aspect ratio and you'll get the diameter in inches.  Actual tire sizes may vary slightly from manufacturer to manufacturer.  BF Goodrich has very good information in Adobe Acrobat (.PDF) format at their site.

Tire Diameters for 14" Wheels

Tire Diameters for 15" Wheels

Tire Diameters for 16" Wheels

Tire Diameters for 17" Wheels

Tire Diameters for 18" Wheels

© GEN 2 CAMAROWORLD 1970-1973