historical accuracy, was: <VV> Tire gurus

Bruce Schug bwschug at charter.net
Tue Mar 29 10:30:35 EST 2005

On Mar 28, 2005, at 12:20 PM, airvair wrote:

> All this discussion brings up a point. With regard to historical 
> accuracy, is it better to preserve the factory LOOK of the (factory 
> stock) car, or it's speedo's mechanical accuracy? In this case, is it 
> better to have a tire with the diameter that LOOKS like an authentic 
> 7.00-13, or one who's size is chosen merely to allow the speedo to 
> function accurately? And if radials end up adversely affecting the 
> look, should they be banned in stock classes?
> As stated before, I dislike the undersized look of (most all) present 
> radial tires. Like any car, wide-open wheel wells (what you get when 
> the tires don't fill the openings) don't look good. The downside is 
> that with 13" wheels, it looks like we are pretty much stuck with 
> undersize-looking tires (when they're radials.)


I haven't seen any other opinions posted about this, so I'll give you 

The CORSA rules for tire sizes were written when most Corvair people 
were under the misconception that a 7.00-13 was 24.1" in diameter and 
the 6.50-13 even smaller. This misconception apparently came about 
because of a misunderstanding between "loaded" diameter (twice the 
loaded radius) and actual static diameter. As a result of the research 
I did about ten years ago plus the "discussion" of this topic that has 
gone on in here, I think most people who have paid attention to the 
subject now understand that this was incorrect. Even my staunchest 
opponent emailed me privately one day and told me he had figured that 
the original 7.00-13 was just about 25.3" in diameter, something I had 
been trying to explain to him for years.

There still exists those who believe the CORSA rules should be set by 
tire manufacturer's charts of "replacement" sizes. My opinion of these 
charts is that they are meaningless because they typically refer to 
load ranges, which to me, don't mean anything when it comes to 
specifying a modern tire that is the true equivalent to the original 
bias tires.

When we discuss this issue, some turn it to a matter of speedometer 
accuracy. To me, speedometer accuracy has nothing to do with this 
issue. I know how to get an accurate speedometer if that's what a 
person is after.

The essence of this discussion is simply, which modern tire LOOKS LIKE 
an original Corvair tire. To me, this has to do with static diameter, 
nothing else. (Obviously whitewall width too, if we are referring to 
whitewall tires.) With this in mind we can study the diameters of 
original tires and modern tires and easily and accurately conclude that 
the 185/80-13 is essentially the replacement for the 6.50-13. The 
6.50-13 varied quite a bit in diameter and the 185/80-13 falls within 
this window. Unfortunately, as you know, there is no modern equivalent 
currently being produced for the 7.00-13.

Regarding the CORSA rules, I think the current rules which allow, even 
REQUIRE tires much different than the original tires make a joke out of 
CORSA Concours competitions. This is like going to an AACA meet and 
seeing modern radials on classic cars such as Corvettes, forty-Fords, 
GTO's, etc. Can you imagine such a thing? It just wouldn't happen. 
These cars are required to have the original tires if they are 
presented as original cars.

CORSA has several different classes for cars depending on how stock and 
original they are. Since I'm not a concours guy and don't want to get 
my CORSA Concourse rules out of the drawer, I'll just refer to the 
class as more stock and more relaxed classes. We won't discuss the 
modified classes - obviously any tire can be used by those 
participants. But in the most stock classes - I think they're factory 
stock restored and unrestored, I think original tires should be 
required. Because original 6.50-13's are readily available this isn't a 
problem for those cars with that size. For those requiring 7.00-13's 
it's a different story.

I think Corky Coker or some other vendor would make 7.00-13's if 
someone would buy them (they don't buy them because CORSA doesn't 
require them). I think it would take a rules revision like this to 
force the issue. I'd give cars a grace period to meet the new tire 
rule, maybe three years. That would give Corvair people time to 
persuade someone to make these tires. Maybe someone like Cal Clark or 
Lon Wall would have to put up some money and buy a quantity of these 
new 7.00-13's to get the job done. Unfortunately, if this tire were not 
available, there would not be any of those cars in the more stock 

I think the relaxed stock classes could be allowed to use modern 
equivalents. The 185/80-13 would fit the bill for replacing the 
6.50-13. As mentioned before, there is no modern equivalent for the 
7.00-13. The only solution I see here is to allow the 185/80-13 as a 
substitute replacement for the 7.00-13 too. I see no reasonable way 
around this. The reproduced 7.00-13 could be used on these cars too, 
but if the owner wanted a modern tire, the 185 would have to do. I 
don't have any illusions of getting a tire company to make some 25.3" 
diameter 195/80-13's.


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

CORSA member since 1981

'67 Monza. "67AC140"

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