That model has an electronic tach, which is triggered by the same electrical impulse that triggers the ignition coils. That says to me that even if your tach was originally fitted to a bike with different final drive ratios it should still work correctly. It also tells me the tach is more of an electronic instrument than a mechanical clockwork (like the speedo).

I'd send it off to one of the advertisers that does "speedometer repair" and have both instruments cleaned and serviced.

You do, indeed, have an electronic tach. It has a circuit that takes a signal from the coil primary and generates a one milliamp (.001 amp) per 1000 RPM output signal. The tach is an ammeter, though built like a fine rugged watch. The tach circuit has a potentiometer for calibration on the circuit board. I checked the calibration on my tach ('95 R100RT) using a signal generator and an ammeter. All of the electronics were OK; I had a mechanical drag problem causing my tach to read low. Do you need any more information on tach testing?

Andy
#15601

I had a similar issue with my 1980 R80. It had a 34/11 final drive, but the tach read higher than it should have and that was most noticeable when I was in the higher gears and above 55 mph. It turned out to be the clutch. I replaced the rear main seal, oil pump cover gasket, clutch and spring. That resolved my issue.
To determine if your clutch is slipping, first ensure it is adjusted correctly. Next, find a hill. Preferably a hill that is on a 55 mph or above stretch. In 4th or 5th gear head up the hill then crank on the throttle. If your rpms go up but your mphs don't then your clutch is either worn or coated with lubricant.
Best of luck .