> So, the original tires for the TR-4a and the TR-4 were 165-15 size. But,
> that was before tire ratio numbers started being used. So a
> 165-15 might be
> anything from about 23" to about 29" (about 585 to 736 mm) in diameter.
I disagree, Darrell. When the Euro-metric system was introduced for radial
tires, all tires were assumed to have an aspect ratio of roughly 82. Of
course, there was some variation in actual tires, actual aspect ratios from
perhaps 75 to 85 have been documented, but the 'official' number was 82 and
the actual variation is much smaller than you've suggested. (Note that even
tires that have a specified aspect ratio, don't always match that ratio
exactly. In effect, there are a limited number of sizes, and a tire is
marked with whichever size it is closest to.) At the same time that low
profile tires were approved for use, the numbering system was modified to
include the aspect ratio, IF it wasn't roughly 82. You can still
occasionally find tires today that do not list the aspect ratio, and safely
assume that it's roughly 82.
For example, my 71 Stag owner's manual lists two original tires, a
185/70HR14 and a 185HR14.
> I can't find anything in my TR-4 manual that specifies a tire
> diameter,
Using the numbers given in the road speed data, we can work backwards to the
effective tire diameter (which after all is what's important). Dunlop (bias
ply) give 501 rpm @ 10 mph, 4th direct & 3.7 gears. This is 812 turns per
mile (which is the way most tire manufacturers give effective diameter) or
an effective diameter of 24.8". Michelin radials are listed as 498, giving
808 tpm or 25.0" dia.
By contrast, Michelin 185R15 Redlines are listed as having a diameter of
26.5"
http://www.coker.com/prodviewer.asp?ID=601&Size=185R15
> The first number is the width of the tread in mm.
Actually, it's the nominal "section" width, which isn't the same as the
tread width. For example, the 205/60-14 tires on my Stag are only 7"
(180mm) wide measured at the shoulders of the tread.
http://www.tirerack.com/tires/tiretech/general/size.htm
> with, in general, better makers
> coming closer
> to the number, and cheaper brands being smaller in diameter.
Hmm, the Michelin 185R15 Redlines are only 26.5", while the Coker 185R15
Redlines are 27.0". Guess Michelin is a cheap brand of tire ! (except the
price of course, $170/each ???)
http://www.coker.com/prodviewer.asp?ID=2909&Size=185R15
> One last size to look at, the 165/86/15. I believe this is a very common
> size to find on TR-3 and 4's today.
Somehow, I don't think so. Can you show me even one example of a tire
marked with an aspect ratio of "86" ? It's not even one of the choices on
Tire Rack's search engine ! (And I'm pretty sure such a tire could not be
DOT approved
Of course, you can buy a nice 165R15 ...
http://www.coker.com/prodviewer.asp?ID=3017&Size=165R15
with an actual aspect ratio of about 77 ...
Randall
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