quote:
Originally posted by bottgers:
I noticed the set of Kumho 795's in the size 185/65/14 have a narrower tread width than the same sized Goodyear Allegras they replaced. I've heard other people claiming this varience between manufacturers of tires even though they're listed as the same size, but I've never actually seen it. Now that I have, it seems strange to me. Isn't there some kind of industry standard tires have to meet? I realize that exact sizing isn't crucial or anything, it just seems like that would be one of the criteria a tire would have to meet before receiving its DOT approval. I'm not complaining by any means. I'd rather have a narrower tire as I'm more concerned about fuel economy and a quiet ride than cornering performance.
The word is "granularity." DOT regulations require that tire widths be specified in millimeters ending in 5. So there is no such thing as a 186/65-14 tire or a 184/65-14 tire. If the first number is 185, then all that means is that on the "measuring rim" the section width (width of the
mounted and
inflated tire at its widest point) is greater than 180 mm and less than 190 mm. The aspect ratio (the second number) is a percentage of height (distance between the rim and the outer tread at the top of a mounted tire) to the section width, again on the "measuring rim." The aspect ratio must be a multiple of five, so there is no such thing as a 185/64-14 tire or a 185/66-14 tire.
Therefore, a "185/65-14" tire could have a section width that actually
measures 180.1 mm with a sidewall height of only 62.6 perccent of that 180.1 mm, or it could measure as much as 189.9 mm with a sidewall height of 67.4 percent of 189.9 mm.
Also, for purposes of setting the tire size, the tire manufacturer states a "measuring rin" size, and rim sizes have a granularity of 1/2 inch. However, most tires have an "approved"
range of rim sizes on which each size tire may safely be mounted. For each half inch narrower than the measuring rim the rim the tire actually is mounted on, the actual section width decreases by approximately 2/10 inch (and vice versa).