I'm not a mechanic and my limited knowledge of this stuff comes from reading through tech manuals that I really may not understand.
The basic gear ratio of all ZF5HP19FL (986 Tiptronic) transmissions is the same. From ZF:
The difference is in the final drive which comes from the bevel gear set that is installed. Here's a view the ZF parts manual pgs 4 & 20 and a
posting of info that came "directly from ZF". I highlighted a bit to help clarify what is related to what:
Still may be a bit confusing if you don't drink as much coffee as I do, but the point is that on the 2.5L 1997 transmission has a final drive ratio of 4.2 whereas the 2.7L 986s have a 4.16 ratio. Different, but significant? I don't know. What is more disturbing to me is that I see the version of the transmission in my Passat has a 3.3 ratio.
...and finally to invalidate my other data is the info from the '03 Boxster Owner's Manual which show different ratios for the final drive. It also has the base listed as an S though so maybe the writer had a bit too much Hefeweizen on his lunch break.
I'm guessing they are both right and I am misreading how the numbers are represented. Ok... that's my post for the day, time for a nap.