Ring and pinion gears., Gear ratios. Options

[Keith914]

What are the practical (including cost) options to change the ring and pinion gears to "lengthen" all the gears so as to optimize a more powerful engine?

[Steve]

AFAIK the 901 gear box only supports a 7:31 ring and pinion. 2nd gear is fixed to the mainshaft, unless you swap it out for a 904 mainshaft, which is overly priced on ebay all the time. With the 904 mainshaft you could change all the gears to get where you want to be. There are tons of gear charts out there. If your worried about breaking first, I have seen some rare 911 4 speeds, that only use 2nd -5, but the gears are more optimized for a four speed. The v8 and 3.6 guys just start out in 2nd and run a flipped H for 5th.
When it comes to the overpriced and harder to shift 915 gear box, from 72-73? It has a 7:31 ring and pinion. From 74-86 it has a 8:31 ring and pinion. I'm running an 86 915 gear box with my 3.2. I only changed 5th to the highest 29:21 for over drive on the freeway.
With a 29:21 5th. I can do 80 mph at 3k RPM's.
Here is a link to a 901 gear calculator. If there is a virus in it and it blows up your computer or innocent animals suffer, you've been warned.
https://cisco.box.com/s/9iw994c3qanzdmij21evrr1bghd9uuhd