[alfa] Re: alfa-digest V10 #204 - Formula for calculating correct tire size

[AlfaNeely@xxxxxxx]

Biba,

    The 165 / 14 is really a 165-78-14 or a 165-80-14, depending on the 
manufacturer.  The 165 / 14, the 185-70-14 and the 195-60-15 are all pretty close 
to the same diameter.  Those were the stock sizes Alfa put on 105/115 Alfas.
    The 195-65-14, the 205-60-14 and the 225-50-15 are also close to the same 
diameter of approximately 24 inches, theoretically.  My personal favorite is 
the 205-60-14 for any Alfa with 14 inch wheels.
    The 155-15 that the Duetto and earlier Alfas came with is about an inch 
taller.  A good replacement for it is a 185-65-15.  That much wider tire will 
fit on a 4 1/2 inch rim.
    I once measured a bunch of tires to compare with the theoretical.  I 
learned two things.  One, tire sizes as indicated by the markings on the tires are 
an approximation only.  They vary from manufacturer to manufacturer.  Hoosier 
race tire seem to be the largest tires of any given size, for what it is 
worth.
    And two, you cannot measure the rolling circumference of a tire with a 
steel tape measure.  You need a cloth tape or a string and a steel tape.  
Measuring the circumference is easier and more accurate than measuring the diameter, 
then divide by pi.

Ciao,
Russ Neely
Oklahoma City



In a message dated 7/8/2004 5:10:26 PM Central Daylight Time, 
owner-alfa-digest@domain.elided writes:
Date: Thu, 08 Jul 2004 14:47:35 -0700
From: alfacybersite <acs@domain.elided>
Subject: [alfa] Formula for calculating correct tire size

Read Jeff Greenfield's post first. Yeah, I thought S and SR sure looked 
like speed ratings. I also figured the original sidewall height of the 
'69 Spider's sidewall was probably 70. If that is the case, how come the 
calculator gives 23.09" diameter for a 165/70-14 (presumably new) tire 
and the well used 165/SR-14 tire I have measures 24"? Or are these 
measurements purely arbitrary and one manufacturer says, "Hey, let's 
make our 165/?-14 tire 1" taller than our main competitors?" What owner 
ever knew if he had a 70, 78,  80 (or?) series tire in the "good old days"?

Biba
Irwindale, CA USA
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