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

[Bruce Schug]

On Jan 7, 2005, at 12:39 AM, BobHelt@aol.com wrote:

> Actually, in revolutions per mile the 175/80R13 almost exactly equals 
> the
> 6.50X13
> 2-ply tire. It's 863 vs. 864
> So it is an exact replacement.
>

Bob,

I'm not sure if you're trying to get me going again or if you're 
serious with your remarks.

You know as well as I do that revs/mi varies quite a bit among tires 
because of variances in size and construction. I have never seen any 
revs/mi figures for actual 6.50-13 tires. Where did you get the 864 
figure? Is this a Chevrolet spec? If so, you know it's just a spec, not 
a figure from an actual tire.

When I did my research, I was unable to come up with any rev/mi 
figures, because the manufacturers didn't publish them at the time. 
They told me they had no such information available. To be more 
specific, in a letter to me dated March 24, 1993, Mr. Ken W. Ball, 
Senior Manager, Sales Engineering, for Bridgestone/Firestone Tire Sales 
Company, wrote, "At that time, we were not publishing revolutions per 
mile. As you stated, bias ply tires had a tendency to grow in dynamic 
loaded radius as speed increased, so tire RPM was difficult to publish 
due to speed variables." No other tire company I contacted was able to 
give me information on this either. The Tire and Rim Association does 
list a spec of 860 rev/mile for 6.50-13's, but again, that is just a 
spec, not figures from actual tires. Their spec also lists the diameter 
as 24.7".

Again, you know as well as I do that because of the variances in rev/mi 
figures that just because one 175/80-13 lists 863, that doesn't mean 
anyone can make a blanket statement that all 175/80-13's have 863 
rev/mi.

Finally, if you've been following this thread you know the original 
question didn't have anything to do with revs/mile but rather about 
diameter. Here's the question:

> I am looking for replacement tires for my 63 Spyder. I would like to 
> have
> a radial with a 1" whitewall. What size tire will look best - and 
> close to
> stock for this car?
>

In my reply I said:

"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."

But this was not the question - the writer said nothing about matching 
revs/mi or speedometer/odometer accuracy.

I'm sure you know my diameter figures are correct, since you finally 
agreed with me that a 7.00-13 was about 25.3" in diameter. My figures 
show that a 6.50-13 is about 24.4-24.6" in diameter. Perhaps we could 
round or average these figures and settle on a "norm" of 24.5" for the 
diameter of the 6.50, realizing that in actuality the varied at least 
from 24.1" to 24.7".

I know that you know that a 175/80-13 is, mathematically, 24.02" in 
diameter and a 185/80-13 is 24.65" in diameter.

Therefore, I'm sure you would agree that the correct answer to the 
question "What size tire will look best - and close to stock for this 
car?", is a 185/80-13.

Regarding "replacement" sizes, I really hate that term as it isn't 
definitive as to what the intention is. As I pointed out in my post, at 
least some of these "replacement" recommendations that came from the 
tire manufacturer's charts had nothing to do with the size of the tire 
and only took load rating into consideration. This is how these charts 
end up listing such sizes as B-50 and B-60 as "replacement" sizes for 
6.50-13's.

Bruce

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

CORSA member since 1981

'67 Monza. "67AC140"