I found this .xls spreadsheet while surfing the web last week.
I'll try to attach it but if it doesn't work,you can go to Instant-G's website and down load a copy for yourself.
In a nut shell,you plug in your gear ratios,tire dia's,& axle ratios,and in each gear 1-thru-5, it'll calculate RPM's,top speeds at each shift.Pretty cool.It'll work with 901 boxes to.

Ron

I have a file for both the 901 and 915 tranny with all the gears available that I know off...try these. Let me know if you need some help using it...I added to comments to the areas that need to be filled in by the used.

-Britain

901 Tranny Calculations

915 Tranny Calculations