Hi folks, I am new to the world of BMW I had the RT since last August and im loving it. I need to find ways to improve the economy as fuel prices keep rising. I understand fitting the bevel box from the RS is a good starting point. How do i identify a RS bevel box are there any numbers on the part? Is this mod just a straight swap or does it involve some other parts? Any other ideas for improving economy would be useful too as im currently getting around 40mpg in normal use. Not sure what sort of mpg i should be getting , it is an old bike and its history is unknown.

Regards

Shaun Hinchliffe

Greetings Shaun Welcome to the forum.
The rear drive from an RT is a ratio of 32/11 which is 2.90 and the RS is 31/11 which is 2.81.
The numbers 32 11 or 31 11 will be stamped on the rear drive aluminium case where the drive bolts to the swinging arm on the upper flat section right where they join together. You don't need to unbolt the drive.
I would expect to get better consumption than 40mpg most I know are getting around 50mpg or better.
It may pay to check out the state of tune before going to the trouble of changing the rear drive.
On the portal page of this site there is a thread called You bought a K100 so what now! It will pay to have a good read of that first and you will get an idea of what to do to get the bike back in good running order.
If you put an introductory post in the new members page you will be able to get access to the downloads page which is a gold mine of great info.

Welcome Shaun!
Just have in mind that the number of splines did change from 16 to 20 from 02/86, at FD end of the driveshaft.
The earlier one (as you got) is more durable than the later.

About fuel economy, tire pressure affects more than one would think.
The recommended pressure in the manual is based on the tire technology that was valid 25 years ago.
Modern tires needs higher pressure to function properly, recommended
pressure these days seems to be 2,9/3,1 bar (manual: 2,3/2,5).

Inge K.

I fitted a RS final drive to my RT last year, and it makes no difference to fuel consumption. I didn't fit it for economy, the splines were knackered on my old one and the RS one just happened to be the first available. Changing for fuel economy alone would not be worth it.

Shaun Welcome My K75 regularly got 21kpl with its 3.2 to 1 FD I grabed a 2.91 to 1 RT FD from a spare bike when I danaged my splines and it gave me 21.5 and sometimes 22kpl (kilometres per litre) that on the whole was 1/2 a k per litre for the change. what I did notice though was the difference when dawdling around i now required a lot more clutch management often in rally traffic or xmas toy runs constantly feathering the clutch to stay slow with the group. If you go the lower FD gearing what you experience now will be increased maybe by as little as 1/2 a klick per litre only, you wont ever reach for another gear when running in top thats for sure but milage may not vary that much. You might have to change these values to mpg to get any real use from them. Keith