do lower profile tires affect speedometer reading

[theringmeister@triad.rr.com]

> Message: 6
> Date: Wed, 9 Nov 2005 08:03:48 -0500
> From: <mboucher70 at hotmail.com>
> Subject: Do Lower Profile Tires affect Speedometer Reading
> To: <quattro at audifans.com>
> Message-ID: <BAY105-DAV981C75014C75F0891066BC1670 at phx.gbl>
> Content-Type: text/plain;	charset="iso-8859-1"
> 
> I'm looking at a pair of 'performance winter' tires.  The size is 
> 195 55 R15.
> 
> My car's original equipment is 205 60 R15.
> 
> I've read that for winter you should change the sizing by one each -
> that is either
> 
> 205 60 R15 or
> 195 65 R15 or
> 185 70 R14 if load capacity allows.
> 
> I'm a bit confused by this rule.  I understand that the tire 
> profile change is ofset by a rim size change.  So then, wouldn't 
> 205 60 R15 and 195 65 R15 give different speedometer readings, 
> since the rim size is the same but the profile is different?
> 
> Bottom line question is will moving from 205 60 R15 TO 195 55 R15 
> work on my car (1990Audi100) or will it change my speedometer 
> readings?
> Other question, where is a good place to get inexpensive winter 
> rims (Wal-Mart, etc)?  It uses 6Jx15 rims.  Are these standard 
> rims, or specialized Audi rims?  I've noticed that most Audis use 
> the 5-bolt tires, but mine uses a 4-bolt.
> 
> Thanks
> 
> Charles
>

Charles, 
Short answer:  only if you change the overall rolling circumference of
the tire/rim combination.

Long answer:  You're using a narrower winter tire (195mm vs 205mm) but
the profile (ratio of tire height from the inner (rim) edge to the outer
(tread) edge is greater (65% vs 60%) so the overall circumference change
from a 205/60/15 to a 195/65/15 is negligible, and should not noticably
affect the reading of your speedometer.
(an exercise in 10th grade geometry class is required here to do the
actual mathematical calculations, but basically the circumference (which
ultimately affects your speedometer) is calculated as C = 2 x pi x r
(the radius being affected by the profile of the tire, which is
proportional to the width of the tire in mm.))

The narrower/taller tire is preferred for snow/mud terrain as it will
not travel on top of the snow as easily, as more weight is focused on a
smaller area, and will dig in and provide maximum potential for grip.
(Think ice skate blade in theory, although the blade is what allows you
to move as the ice melts under it from the pressure/heat generation).

Wylie Bean
theringmeister at triad.rr.com
90 cq
99.5 A4 Avant