Why do tires vary in size?

Messages
454
Location
Cheyenne
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.
 
Messages
15,410
Location
Santa Barbara, CA
ive noticed that to a lot. for 03/04 mustang cobra owners, which have 17x9" wheels all around, you can fit a Nitto 315/35/17 on a wheel that came with a 275/40/17 no problem. however if you try to fit lets say a 315/35/17 BFG Drag Radial, it wont fit and requires a 10x5" wheel. usually the name brands, Michelin, Goodyear, Bridgestone and their higher end subsidaries run correct sizing, while the cheaper brands, Cooper, Kumho, run smaller. this just what ive seen from mounting a few thousand tires.
 
Messages
12,385
Location
Northern CA
In addition to what BDCardinal said, tire size refers the width of the tire at it's widest point when installed on the nominal with rim that it's rated for and it's diameter. Those dimensions aren't necessarily exact and AFAIK, there is no standard on treadwidth vs tire size so treadwidth is wide open.
 
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3,939
Location
Somewhere in the US
Since there hasn't been a technical explanation: 1) If you make the tire smaller in width and / or diameter, you use less material = less cost to make. Downside: The smaller the air chamber, the more stressed the tire materials are = worse ride, worse handling, and higher failure rate 2) If you want a tire that is good for fuel economy, the less material in the tread, the better (and a narrower tread width is one way to achieve this.) Upside: Good hydroplaning resistance, good snow traction (in most cases). Downside: worse dry traction, and worse wet traction where hydroplaning is not involved.
 
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).
 
Messages
5,570
Location
New Zealand
I noticed this a few years ago too,it was mainly the European tyres,but then the Korean's Kuhmo and Hankook started doing it too....the older style wider and flatter tread is becoming rare. Euro's are more fussy about tyre safety as their open road speeds are higher,so I can't see them doing a backwards step here,certainly the tyres seem to work OK. I fitted Barum 185-70x14 tyres to my diesel Nissan van specificaly,because it has no power steering and the narrower tyre would give lighter steering.That was over 50,000km ago and I've been happy with them.
 
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