Kay,

Just under the "MPH" on my '85 GL is "805".  It's supposed to be the
number of revolutions the speedo-cable makes per mile traveled.
5280'/805revs=6.56'/rev or 78.71"/rev.  A tire with a circumference of
78.71", or a diameter of 25" (diameter=circumference/pi) should give an
accurate indication of speed.  My 195R75x14 Dunlops measure 24.375".  I
figure I'm indicating around 3% faster than actual speed.

Does anyone out their have a different number on their speedos?

Bob - WA4RRN
'85GL