I keep seeing this when you buy tires you can get 31" 33" 35" ect then some times it saids 285 305 315 is this the same thing just a different way of saying it if so how do you convert it? EX what dose 315 stand for?????
Thanks in advance Tjsport07
Tread width = exactly as it sounds. The tread section width in Millimeters. In our above example 315 mm
Profile or Aspect ratio= Is the sidewall height expressed as a % of tread width. Using our above example the sidewall height is 75% of 315mm or (.75*315) or 236.25mm
Rim Size = the standard rim size in inches.
Okay so now I have all that info, how do I convert my 315/75/15 to 31s 32s 33s etc?
First you need the conversion factor. 1 inch = 25.4 millimeters.
Now our 315/75/15
The first number in a standard tire size is the height. To find the height in a metric size you need to do a little math.
You know the sidewall height (236.25 from our above example) and there are 2 sidewalls to count. (1 between the ground and the rim and one between the rim and the top of the tire)
So Tire height =
236.25mm
236.25mm
+15 inches
or
472.5 mm + 15 inches
Now convert 472.5mm-> inches
472.5/25.4 = ~18.60 inches
So tire height equals
18.6+15 = 33.6 inches
the width of a standard size is the middle number.
We already know this from the first number of our metric rating we simply convert.
315/25.4=~12.4
The third number is the rim size (unchanged)
So a 315/75/15 "equals" a 34/12.5/15
*One final word on tire sizes though. These will vary dramatically between brands. When I worked in a tire shop in college there would be as much as 2 inches of difference between two tires of identical size and 2 different brands.*
