So how do you calculate the diameter of your tire based
on the letters and numbers printed on it?
Basically all tires have something like this printed on it:
P315/70R17
And if we label all the number above like this:
Type Width / Ratio Radial Rim
Then the tire diameter formula is as follows:
(((Width x (Ratio/100)) x 2)/ 25.4) + Rim = Tire Diameter
However, if you don't have a calculator handy, you can simply fill in the information here and
we will calculate the tire diameter for you:
Here is a more detailed explanation of the different parts printed on the tire:
Type: Tells you what kind of tire it is:
P = Passenger vehicles tire (most cars and trucks)
T = Temporary tire (smaller tire)
LT = Light Truck tire (larger SUVs, pickups etc)
ST = Special Trailer (boat trailer etc.)
Width: This number tells you the width in millimeter of the
inner walls of the tire. (you can devide millimeter by 25.4 to get inches)
Ratio: This number tells you the distance from rim
to tread as a percentage of the width above.
R = Radial (you can sometimes also find D for Diagonal and B for belted)
Rim: The last number indicates the inner diameter and
should match the wheel and range from 9 to 28.



