trenace
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posted August 20, 2005 11:58 PM
 Edited By: trenace on 21 Aug 2005 01:27
The definitive answer is that tire manufactures do not take these measurements literally. A "180" does not mean a width of exactly 180 mm, nor would it necessarily mean that the tire is narrower than another "190" from the same manufacturer.
Also, the aspect ratios are only approximate. The manufacturers don't consider themselves beholden to using values of only exactly 50.0 percent, 55.0 percent, etc but for marketing, the aspect ratios actually used are usually rounded to the nearest five percent.
As a result, your statement "a 180/55-17 tire is taller than a 190/50-17 tire" is not necessarily so.
The figures are guides at best and cannot be used as you are trying except as really rough guides that could easily be off a half-inch in diameter or perhaps even more.
For example, a Bridgestone BT-014 in 180/55-17 can be 0.6" smaller in diameter than a Pirelli Diable 180/55-17.
Even if one's goal is so simple as to purchase again the same size tire as previously, it's not so simple as just purchasing a tire of the same model with the same numbers from the same manufacturer again.
For example if you change what was a generic aftermarket Bridgestone BT-014 in 190/50-17 for another BT-014 in 190/50-17, but the guy pulled one off the shelf that actually was intended for a CBR-1000RR rather than being a generic, the tire will be 0.3" wider, actually the same width as most 200/50-17 tires though it's called a 190.
Fun, isn't it?
  
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