On Mon, 14 Jul 1997 11:56:19 -0400 larry.g.unger@lmco.com writes:
>> It is very possible that two tires from different mfg. will have a
>> different tread width on the same nominally sized tire. For a
>> very simple reason. The tire size has nothing to do with tread
>> width.
>>
>> The 175 (or 185 or 165 or whatever) refers to the tire's maximum
>> cross section when mounted on the rim width for which it is
>> designed. When that tire is installed on a wheel with a larger or
>> smaller rim width, the cross section will vary, too. A good number
>> to go by is for every 1/2" change in rim width (smaller or larger),
>> the cross section will change by about .2" (5mm). Thus if you take
>> a say, 185 tire that was sized based on a 5" rim width and mount
>> it on a 51/2" rim, it will actually be closer to 190mm wide. (but
>the
>> tread width doesn't change).
>
>Understood ... sizing is based on a specific rim width ... the tread
>width is the tread width ... width of the cross section varies with
>rim width ... how about the diameter? IOW, a 195/65R15 Dunlop
>D40 M2 was sized for a 6.5" rim, has a tread with of 6.5" and a
>diameter of 25.1" ... will the diameter vary if I install it on say
>a 5.5" rim?
>
>Safety Fast! ... larry.g.unger@lmco.com
>'61 MGA 1600 MkII
In theory, yes. But for practical purposes the change in Outside Diameter
is so slight as to be safely ignored. As it was explained to by (by a
Michilen engineer neighbor, who is getting scarce when he sees me coming
with another question!) the change in width is a function of the change
in rim width, and with the same or near same tire pressure, the OD will
not change measurably. In fact, the minor change in OD would be less than
the manufacturing tolerance of any two tires off the assembly line.
Hope this helps
Rick Morrison
72 MGBGT
74 Midget
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