Technical Forum (Read Only) > Wheels and Tires

Converting Metric Tire sizes to Standard (reading Tire Sizes)

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Captkaos:
you can covert to metric by taking the following 285/75/R15
285 is the section width (not tread width)
75 is the aspect ratio - percentage of the section width equals side height
15 is the rim diameter.

If you take 285 divided by 25.4 = 11.2" that is the section width in inches
Multiply 11.2 x .75 (aspect ratio) = 8.4"
Multiply 8.4 x 2 (top and bottom of sides) = 16.8"
Add the 15" to the 16.8 = 31.8" overall diameter from top to bottom.

If the size is listed as P285/75/R15, this means it is the American P-Metric rating.  The only difference in a P-metric and the Euro is the load capacity and inflation specs

If there is a number AFTER the above like  P285/75/R15 87S, the 87S is the Service Description.  The first number is the load rating, the second is the speed rating.  87 = 1201lbs and S = 112 mph

If the designation begine in LT285/75/R15 this means it is a Light Truck tire for high load capacity.

Numeric numbering is pretty self explanitory.

31x10.50x15 is a 31" tall tire with 10.50" Tread on 15" wheels.