OK, so just sort of thinking out loud here. We've all seen the discussions about putting a GL1200 drive on a KZ1300 to lower cruising RPMs. I don't have an issue with this, but working out drive for the wheel, shock mounting and a drive shaft requires a fair amount of fabrication.
But, it seems to me it "should" be more straight forward to swap the ZN front bevel gears into a KZ (though it's work as it means splitting the cases). These are the bevel gears between the transmission and the drive shaft flange.
I've never had these ZN parts in my hand, so not sure this would work, but...
The 1980 KZ is listed as having 20/24 (0.83) tooth bevel gears and 35/11 (3.18) final drive. The ZN is listed as 19/26 (0.73) and 34/10 (3.4). So overall the ZN has an effective 5th gear drive of 4.26 vs the KZ of 4.55. This is reflected in the specified top speeds (KZ=139, ZN=145).
So at 70mph your KZ should be at about 4050RPM. If you had a ZN, you should see 3800. If you had ZN Front Bevel Gears and KZ Final Drive, you should see 3550RPM, and a theoretical top speed of 158 @ 8000RPM.
The GL1200 Final Drive swap should net you 3600RPM @ 70MPH and 155 @ 8000RPM. So effectively the two swaps have almost the exact same effect.
Now I would not dream of splitting the cases for this unless I was at least pulling the top end anyway.
Anyone happen to have a set of ZN and KZ gears to compare?
I too looked at this thinking it may be a possibility after looking at Trikebldr's post of his GL rear drive mod. I thought that was too much work to achieve a 500 rpm drop. I would like the rpm drop just to give my ears some relaxation.
The change if possible would make a 14% decrease in rpm. to achieve this through tire size would require a change in diameter of almost 4 inches. Without lengthening the swing arm and adopting a larger rim but just looking at tires only by going to a 240 tire can you increase the diameter by 1" so I'm not sure how you could go to a larger diameter wheel and still meet the load rating.
The topic definitely needs more couch potato engineering.