Posted by David on April 11, 1998 at 08:23:43:
In Reply to: Just my imagionation or what? posted by OC on April 07, 1998 at 20:47:56:
Perception vs Imagination - robmoz, please excuse my correction but you missed the mark slightly (this is not intended to start another round of volley's). Though your explaination does have merrit, and I also believe OC means tread width not diameter, all things you explained are correct except for the fact that tread width is tread width. OC said that the tire's on his prelude "appeared" narrower than those on the Z24. The 6'' vs 6.5'' rim width rational you explained in relation to tire fit on the wheel is correct, but would have the opposite effect of what OC said he saw - the tire's mounted on the 6.5'' wide wheels, given that they are the "same", would appear to be wider. OC, it is not your imagination.
Let me explain
It is two separate but related issues. First, the style or design of the tread itself can cause this difference in "appearance", though it could me measurable and not just visual - depending on design - how far on to the shoulder the tread pattern is taken (towards the sidewall), if it is a "hard" or squared off edge or if it is rounded, "soft" rolled egde treatment or transition from tread pattern to sidewall. Although this is indeed a visual or percieved impact it can as well be measured.
Second - and without looking it up for these two tires, is probably the most common difference. the size nomenclature - 205/50 or /55R16 - 205 does, as you stated robmoz, does indicate the width, but actually the section width not the tread width. section width is distance measured from sidewall to sidewall. If you drew two parralel, verticle lines up from opposite sidewalls it would be the measured distance between them, different from the tread width. Although the section width is standard, as you mentioned, there is an allowable and used variance. Basicly the actual measured section width of any given size can be + or - a certain % of the indicated (205) section width. One manufactuer could build a 205 that would measure more that 205 mm and another could build one that measured less that 205mm and they both would be called a 205/?R?.
You are correct that perception and visual appearance play a role, especialy with so many variables - rim width, aspect ratio as a % of section width, etc. Though at the same time actual differences are a reality, even with tires of the same size nomenclature.
If you would like to verify my explanation just check out a couple specification and data charts from different manufactuers and compare all the dimentions of the same "size" tire from two different manufatuers.
Sometimes your eyes don't deceive you :) !!!