<VV> Tires for a '63 Spyder (Here we go again)

[Bruce Schug]

On Jan 5, 2005, at 9:53 PM, Wrsssatty@aol.com wrote:

> In a message dated 1/5/2005 9:46:52 PM Eastern Standard Time, 
> bwschug@charter.net writes:
> I'm sure the 175's work fine on your '63, but they are small.
>  185/80-13's are very close to the diameter of the original 6.50-13's.
> Well, larry "Stock Is..." claypool wrote to me; "hi bill the correct 
> size for an early model is 175-80x13; the 185-70's...are actually a 
> bit smaller in o.d. than the proper size;...185-80...are 1 size 
> bigger."
>  

Well...

Original 6.50-13 tires ranged from 24.1 to 24.6" in diameter. This 
covers a variety of tires documented by Bridgestone/Firestone. Corky 
Coker currently reproduces a U.S. Royal 6.50-13 tire that is 24.58". 
His "Coker Classic" is this same diameter. I am not aware of any other 
6.50-13 tires available.

The Tire and Rim Association listed the diameter of a 6.50-13 as 24.7". 
This is a sort of "standard" but does not necessarily mean that there 
were any 6.50-13's that big, although there probably were. Most of the 
tire manufacturers I contacted about this in 1993, didn't have records 
of these tires.

With such a wide range of diameters it is hard to come up with a set 
diameter and say that this is what they were. But most of these tires 
were 24.4-24.6, so that's what you need to shoot for in a replacement.

Let me state that you have to decide what you're trying to do when you 
pick your tire size. If you're looking for an accurate 
speedometer/odometer, then what you want to do is to find a tire with a 
revolutions per mile figure that matches that on your speedometer. 
Remember, automobile speedometers/odometers weren't accurate from the 
factory, but if that's what you are trying to accomplish, this is how 
to do it.

Also, many times when you ask for a "replacement size" what you get is 
a tire with the same load range. Its diameter may be significantly 
different. We have all seen the "replacement" charts in tire catalogs. 
One such chart shows the following as "replacements" for 6.50-13's: 
B50-13, B60-13, B70-13, B78-13, 175-13. I think you can see these tires 
are very different in diameter.

Your question was, "What size tire will look best - and close to stock 
for this car?" This is what I'm trying to tell you.

Here are some diameters of popular 13" tires:

185/80-13 - 24.65"

175/80-13 - 24.02"

205/70-13 - 24.30"

195/70-13 - 23.75"

185/70-13 - 23.20"

The conclusion I draw from this information is that a 185/80-13 is very 
close to an original 6.50-13. Do you agree? Keep in mind, these are all 
mathematical diameters, not actual diameters. You need to look at the 
manufacturers specs for actual diameters. I got these figures from the 
following web page, which saves me the trouble to doing the math: 
http://ejelta.com/tiresize/index.html?ws13=1&

So, if this is true, then why is it that many Corvair experts have 
different recommendations? Why is is that the information in the "CORSA 
Tech Guide" as well as the CORSA Concours Rules differs?

After much discussion on this subject with other Corvair folks, it 
became apparent that it was all based on a misunderstanding. 
Previously, Corvair folks had taken the revs/mile figure on their 
odometer and mathematically calculated a diameter from it. For years 
the rule of thumb was that a 7.00-13 was 24.1" in diameter. This is how 
that figure was arrived at. But tires don't work that way - THEY FLEX! 
This means that the actual diameter of the tire must be bigger, because 
the part of the tire that meets the road is LESS than one-half the 
diameter of the tire! Some might say it is the radius minus the 
deflection.

Looking at dimensions of actual 6.50 and 7.00-13 tires, this allows us 
to understand that when these tires were mounted on our Corvairs and 
driven down the road, the speedometers/odometers were as close as the 
factory intended. This explains why the diameters of actual 13" tires 
makes sense with correct mathematical theory. Understand?

So, why then is the information in the previously cited CORSA documents 
inaccurate? Simply because they have not been corrected. It is my hope 
that someday someone within CORSA will correct this and recommend 
correct sizes for tires in the concours rules.

By the way, using this same correct logic, the diameter of a 7.00-13 
was about 25.3". As you can see, there are no tires currently available 
that are close to this. Further, I am not aware of any original 
7.00-13's being reproduced. If you call Corky Coker and ask him why he 
doesn't reproduce a 7.00-13, my guess is that he'll tell you because 
there's no market for them. My opinion is that there's no market for 
them because CORSA doesn't require them in stock/original-type concours 
classes. It's a chicken and egg thing. Perhaps some day this will 
change. In the meantime, next time you're at an AACA show, see if Model 
A's and '40 Fords and '55 Chevys and GTOs and Mustangs have the 
original tires on them.

Bruce

Bruce W. Schug
CORSA South Carolina
Greenville, SC
bwschug@charter.net

CORSA member since 1981

'67 Monza. "67AC140"