1987 CRX. Strange tachometer problem? stock crx. Tachometer always at about 2/3 of the actual RPM speed. When I'm doing 40 mph in 2nd, it should be at 3k but the tach is showing 2k. Note: tire size is 14" and tach started doing this suddenly. It goes above 30 mph at 3k rpm in 1st gear according to the tach...but that's what I think it should be at 4.5k rpm.

MPH gauge is tested and correct. I replaced the ignition control module (ICM or igniter). I opened the instrument cluster, checked the connections, and checked the grounds. I got 15V from the alternator when cold and about 14.7V after running for a few minutes, that's with everything off -- radio, lights, air vents, defroster, etc.

Not sure what's going on. The solder points on the instrument cluster looked fine too.
thanks

 

I'm not up on my 1st gen wiring, but I would assume that the circuit board in the tachometer has electrolytic capacitors on it, and they are almost certainly in need of replacement if they are original. If you have access to another cluster, you could try swapping it in to determine if it is an issue with the tachometer itself or something else, or you could just pull your tachometer out, determine what capacitors to buy, and replace them.