Correct Speedo and Final Drive but Speedo is off

[jjwithers]

I have a 32/11 final drive and a W1,078 on a '78 R100S but the speedo is reading about 10mph too fast at speeds over 30mph and the odometer is clocking up the miles pretty fast too.

I am reading that this is the correct speedo and final drive ratio.
I'm uncertain about what 5Speed transmission is in there. It could be the culprit.

Perhaps somebody changed something along the way... like the gearing in one of the two components. Or it is the tranny.
I have W 1,112 and W 1,114 speedometers lying around that I could swap in.

I'm assuming that the W 1,112 would be the appropriate next step up but what could I be risking? Would the speedo get closer to what I am after by reading even slower, or would it be worse?
Any other advice would be appreciated.

-Josh

[Rob]

I think the first thing to do is determine the final drive gearing.
It can be done easily, but you may need to pull the universal boot to count the revolutions. With two people, one could probably try and feel the rotations through the boot while the other person pushes the bike for one tire rotation.

[jagarra]

That speedometer is correct for the ratio, here is a link to Snobums site with the info.
http://bmwmotorcycletech.info/ringgears.htm Maybe something is wrong with the speedo or your FD is mis-identified.

[jjwithers]

Thanks Rob,
To expand on what you said, I found this
"Place a mark on the tire and on the driveshaft. Place the transmission in neutral and spin the tire exactly two revolutions and count the number of times the driveshaft spins. This number equals your gear ratio."

[barryh]

W1078 is the number of speedo cable revolutions the speedo needs to record 1 mile travelled.

So if you were to use a W1112 or 1114 speedo in combination with your 32/11 final drive, they would read a lower speed and record lower miles on the odometer by 3% and 6% respectively.

[Chuey]

Aren't they notoriously optimistic, anyway? I mean, isn't that part of the design intent?

Chuey

[barryh]

BMW's warranty tolerance was 10% + 2.4 MPH. The example they gave in the service bulletin was at a true 55 MPH the speedo would have to read in excess of 62.9 MPH before they would fix it.

[jjwithers]

By this logic, I should be able to put in a spare 1114 speedo and it should 'slow' the odometer and speedometer down a bit.
I have yet to make the time to count the revolutions of the wheel vs driveshaft. Been a busy week.