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From: geoff@denelvx.UUCP (Geoff Baum)
Newsgroups: net.auto
Subject: Re: Tire Questions (info request)
Message-ID: <116@denelvx.UUCP>
Date: Mon, 29-Jul-85 14:28:02 EDT
Article-I.D.: denelvx.116
Posted: Mon Jul 29 14:28:02 1985
Date-Received: Tue, 30-Jul-85 19:40:36 EDT
References: <1839@amdahl.UUCP>
Distribution: na
Organization: Denelcor, Aurora, Colorados
Lines: 24

> 1. The original tires were size 155SR13. The tire dealer said that
>    P175/70R13 are acceptable size replacement for 155SR13.
>    What I am not sure about, is the overall size (outer diameter)
>    of the new tires compared to the old ones ?
>    
The "70" in "175/70R13" means that the tire's sidewall height is 70% of
the tire's width (70% of 175 mm).  This number (the 70) is called the
aspect ratio.  When no aspect ratio is listed, a standard aspect ratio
is implied.  For 13" tires this implied aspect ratio is 80%.  I happen
to have written a program which takes all this into account and calculates
equivalent tire sizes.  It turns out that your original tire and wheel
produced an overall tire diameter of 578.2 mm.  Your new tires (same wheels)
produce an overall tire diameter of 575.2 mm.  This is a difference of 0.5%
in overall diameter.  Since changing one tooth on your speedometer driven
gear (the thing that is changed by the manufacturer to accomodate different
tire sizes) changes your speedometer reading by about 5%, you can see that
you have nothing to worry about.  As a matter of fact, I calculate that
the normal process of wearing down your tires probably changes your
speedometer's accuracy by about 5% also.  So, don't worry and have lots of
fun.  You made the right move.

As for the 82S vs. 82T, I have no idea.

Geoffrey Baum