I'm posting this on behalf of my friend Phil Moser:
This will be of interest to all K100 owners.

I’ve successfully built a 2,54 ratio ( 33 : 13 ) bevel drive into a K housing using the gears from an R1200C. Standard K100 is 2,82 ratio ( 31 : 11 ), and so it reduces revs throughout all gears by 11%.

It’s made mine VERY long legged, slightly under 4k revs at 120km/h. Another advantage is reduced vibrations. The motor, being very torquey, pulls fine through this ratio. The speedo, taking its input from the rear wheel speed ( not drive shaft speed as in airheads ) stays correct, of course,  and the “spacing” of the gears is not affected as it is in “shorter 1st / longer 5th” as is done by a number of airhead GS riders.

But be warned ! Neither a cheap nor an easy job !

For a more comprehensive article with pictures, see https://pdfhost.io/edit?doc=1ca72a28-fe6b-4564-ada4-7c44fe9d73ea

The link takes you to PDF HOST ... you will see a thumbnail of the article ... click on VIEW DOCUMENT

I discussed this with a friend who passed in 2021. He was a wizard with final drives.

You can do it on all the K bikes, K1 K75 [if you want] K100 K100 16v and K1100.

If you do it with early K100 you need to change drive shaft to 16 spline.

I have a final drive I had intended him to do for me but he passed suddenly so its not happened.

You could get it down to 2.75 by converting it to a paralever using a  33/12  final drive from a K1 or late K1100LT.