Audi 90 sedan vs. coupe tire size questions

["Smith, Kirby A" <kirby.a.smith@lmco.com>]

Revolutionary peoples:

According to my Bentley, the 90q sedan uses 195/60-14 tires.  The optional
tires listed for the 90q coupe are 205/60-15, a tire significantly larger in
radius (6.3%).   Also according to my Bentley, both sedan and coupe use 4.11
differentials.  Therefore, it seems to me that the speedometers of these
respective cars should be calibrated differently, even though I have seen no
mention of this in the Bentley, and a scan of my fiche reveals a speedometer
section that differentiates in max speed and in miles versus kilometers, but
not in sedan vs. coupe.

With 195/60-14 tires on my sedan, and an instrument panel labled "7056
imp/mi", I believe (from matching my speed with state police cars trying to
assert the speed limit) that my actual speed is very close to that
registered by my speedometer.  So what happens with coupes?  Do those owners
with the optional Speedline 15 x 7 wheels put up with understated speeds?
(A 205/50-15 would be a closer match if the 195/60-14 is right on.)  And
what is the relation between speed sensor impulses and tire revolutions?  If
you know the answer to any of these questions I would be pleased to have you
respond.  TIA

                .... Kirby   (Kirby A. Smith)
                              2 x 1988 90q
                          New Hampshire USA