I have an '86 GN. The factory tach on the bottom right of the speedo is digital (not a digital speedo). It always reads higher then actual RPMs. It reads close to 2000 at idle and over 4000 at 65 mph. Maybe it's just a faulty tach? Any help is greatly appreciated.
It's probably the most common problem with the boost/tach modules. The two chips need to be replaced and the unit needs to be recalibrated.
Here's the fix:
http://www.casperselectronics.com/cgi-local/store/commerce.cgi?search_request_button=yes&keywords=boost/tach
-John Spina
http://www.casperselectronics.com
Thanks for the help. We should have our part soon.