"Tiny Tears" Teardrop Camping Trailers
Technical Tips
Determining the Outside Diameter of a Tire
9 December 2002
This formula determines the outside diameter of a tire. It is useful for deciding what size tires to use, what size fender you need, or placement of the axle or door so there is no interference.
Tire Size Formula
Tire Dimensions: For this example we will use a tire
size of P195/75R14.
195 is the section width of the tire (how wide the tire is) in milimeters, dimension "X"
in the diagram.
75 is an aspect ratio - from the top of the rim to the top of the tire (the
sidewall, dimension "Y" in the diagram) is 75% as tall in that section as the tire is wide.
14 is the
overall diameter of the rim, dimension "Z" in the diagram.
(In words that is: X times Y divided by 25.4. Mulitply that answer by 2. To this answer add Z.)
Our Example: X=195, Y=.75 (75% converted to decimal), Z=14
195 times .75 = 146.25
146.25 divided by 25.4 = 5.76
5.76 times 2 = 11.52
11.52 plus 14 = 25.52
25.52 inches is the overall diameter of a P195/75R14 tire.