> > Also is the diameter of the 185/15R smaller than the stock TR4A
> > tires? I ask
> > this because my speedometer seems to run a bit high.
>
> My book says the stock TR4A tire was 165-15, so the 185s should be
> substantially taller. Your car may have had the gear ratios inadvertently
> changed, or your speedo head may simply read high. They sometimes do that
> when they get old.
Well...yes, and no... lol
When speaking of tire sizes one absolutely has to take into mind the "ratio"
of the tire also. As an example, a 185-70-15 tire is much taller than a
185-50-15 tire.
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
can't find anything in my TR-4 manual that specifies a tire diameter, but it
does specify tire type and size. Mostly different Dunlop tire lines, the
Gold Seal 5.50-15 or 5.90-15, the RSS 5.50-15 or 5.90-15, or the Duraband
6.5-15. It also list Michelin 165/15X. Before tire ratios started being
used ech brand would build a width, and make it as tall as they wanted, so
that you had to specify tire name and size to get the right ones for your
car.
I am not sure, but I think the Dunlop RSS 5.90-15 had a 25.25" (641mm)
diameter.
So, how does this compare to how modern tires are measured? Modern tires
use a three number system to roughly define the size of the tire. An
example would be a 205/60/15, or a 185/70/15, or 165/86/15.
The first number is the width of the tread in mm. The second number is the
ratio of the sidewall to the tread width in percent. The last or third
number is the diameter of the intended mounting rim or wheel. By having
these numbers and doing a little simple math you can find the aproximate
diameter of the wheel. This aproximation varies in accuracy from
manufacturer to manufacturer, with, in general, better makers coming closer
to the number, and cheaper brands being smaller in diameter.
So, a 205/60/15 has a tread width of about 205mm. There are two sidewalls
in the path when you measure a tire for diameter, so two 60%'s is 120%. You
multiply this 120% (or 1.2) by the 205mm tread width to get the total
sidewall height. 205mm times 1.2 = 246mm. Add this number to the wheel
diameter of 15 inches (381mm) and you have the total diameter of the tire,
or how tall the tire is. In this case about 627mm, or 24.7 inches. And if
I go outside and measure my Kumho 205/60/15 tires on my 1962 TR-4 I find
that they are 24.6 inches in diameter, but then they are not new and have
worn some tread away.
Lets look at another size, 185/70/15. The sidewall ratio is 70%. So, 185mm
tread width times 1.4 (double the sidewall height) = 259mm of sidwall. Plus
381mm for the 15 inch wheel size = 640 mm (or about 25.2 inches)total
diameter. So here the 185 tire is taller than the 205 tire. But both are
within a few percent of the original tire diameter of about 25.25 inches, so
there should be little, if any, noticable tire size induced speedometer
error.
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. The width of 165 mm times double the
sidewall ratio, or 1.72, yields a sidewall height of 283.8 mm. Add to that
the 15 inch rim diameter of 381 mm and you get about 665 mm, or 26.2 inches
of tire diameter.
So, with three tires, a 205/60/15. a 185/70/15, and a 165/86/15, the tallest
tire is the narrowest of the three, and the shortest is the widest. But all
of them are right around, within a few percent, of the original TR-4 tire
diameter.
But, tire diameter is not everything, those wide meats still have to fit
under the fenders and not bind with anything.
On one of my web pages I go very lightly inot wheel offsets and backspacing,
and there are also some pictures of my TR-4 with the 205/60/15 tires mounted
on some very commonly found rims.
http://home.mchsi.com/~token/TR4Page5.htm
Darrell
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