help understanding rpm/ratio

[Nicholas Lawrence]

    When comparing the trans and rear differential between a '90 200q and a
'91 200q I see the '90 listed as having a final drive of 3.89  and the '91
final drive of 4.11. I believe this represents a 5% difference.
Transmission gear ratios are listed as the same.
    Both cars have a speed in fifth gear of 25mph per 1000rpm. And from
actual observation cruising rpm @ 65mph are the same.  I know the
speedometer is off on the '91, (checked by other means) and this is reported
to be common situation.  The tire size on the '90 is 205 and on the '91 its
215.
Could this be the difference?  Am I overlooking something?
Reason for the research, I may need to replace the trans on my '90 and I
have a '91 available.
   TIA
Nick
'90 200 tqa