Lon asked: How do I change final drive rations on a K?
All rear disk brake non-paralever final drives are interchangeable. Ratios commonly go from 3.2 on a K75 to 3.09, 3.00, 2.91 on a K100RT, to 2.81 on a K100RS. Used units can frequently be had for $300-400. New ring and pinion sets are a lot more, like $700, and labor to install is extra, of course. Note: Early K100s have a different final drive spline design, so a new/used driveshaft may also be required.
The only K75 final drive that WILL required a ring and pinion set to change ratios is the early K75C drum brake unit. These only came in a 3.2 ratio.
Newer paralever K final drives are equally interchangeable, I believe.
K Whiner MC#11