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Tire sizes (also tires for sale)

To: mgs@Autox.Team.Net
Subject: Tire sizes (also tires for sale)
From: "Jeffrey B. Lewis" <jblewis@MIT.EDU>
Date: Tue, 19 Aug 97 18:17:39 EDT
I recently had to buy tires for my B as well.  The Continentals that I had
on the car were 165HR14's and they were great.  Unfortunately, 
road construction killed one of the them.  I figured no problem, I'll
just replace a pair and I'd have a better spare to boot. However, as the
original posting noted, Continental no longer makes this size. As someone
mentioned Avon (and also, Dunlop) make this tire, but the price is high.  
Worse yet the local NTB says they will not even deal with tubes!

        So I went about figuring out what modern tire size seems like the
best match to the original 155HR14 and old replacement 165HR14. 
By looking at the spec sheets for a large number of metric tires (such as
the 165HR14), I inferred that the aspect ratio (sidewall height/section
width * 100) for such tires is about 80 (you knew that you say?  Hey, I'm a
PhD student, inferring what practitioners already know is what I do).  That
means the 165HR14 is basically a 165/80HR14.  As anyone who has seen a tire
add in the last ten years will tell you, the largest aspect ratio you are
likely to see is a 70 and many performance tires are lower profile (aspect
ratio of 65, 60 or even less). 

The table below shows the typical diameters of tires of various sizes.

        Size                    Diameter  
        ==========              ========
        155HR14                 603.6   
        165HR14                 615  
        175/70HR14              600.6     
        175/65HR14              583.1   
        185/70HR14              614.6   
        185/65HR14              596.1   
        185/60HR14              577.6   

So aspect ratios of less than 70 are going to be a problem.  The 185/60HR14 
are 6 percent shorter than the 165HR14, that's about 220 more RPM required
at 65 mph--an amount that you're going to notice.  The 185/70HR14 is about
the same height as the 165HR14, however its really too wide for a 4.5" rim
(especially a banged up, not so sound old wire wheel rim).

The rim is not supposed to be less than 7/10ths as wide as the 
tire's section width. For a 4.5" inch rim, a section width of 165 is right
at 7/10ths. A 185 tire puts the ratio at only 6/10ths.  No manufacturer
recommends a 185 tire on less than a 5 inch rim and ideal width 
for 185's is 5.5 inches.  175's are supposed to go on 5 in. rims but 4.5 in.
is ok (according to Firestone).  So the answer?

        Performance: Bridgestone Potenza re92 175/70HR14
        Otherwise: Steal the 175/70HR14s of the nearest saturn.

BTW, anyone want to buy a pair of perfectly good Continentals?

Jeff

Disclaimer: Though I am at MIT, I'm a political scientist. My tire buying
advice may only be worth about what you paid for it.
---------------------------------------------------------------
Jeffrey Lewis                                   jblewis@mit.edu
Department of Political Science                 617.258.5888
MIT E53-436
Cambridge MA 02139


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