I have an original 65 tach that I am putting in my car. I decided to hook it up to another car to make sure it works before installing in my Chevelle and compare the two for accuracy. I do not have my car wired completely and is not ready to start yet. When I tested it on the other car, it did work but the RPM's were off from the tach that was in the car.(about 20% higher reading on the RPM)
I am wondering is there an adjustment in the tach or does this tach need different volts than 12 volts. I have an original wiring harness and I'm thinking it might run through a resistance wire. I would really like to know this tach is accurate before installing the dash bezel in the car. Since it responds well except for being off on the RPM I'm assuming it is either a voltage issue or a calibration problem. Any help would be greatly appreciated.

Any thoughts or ideas. The tach I compared it to was a manual tach.....:)

No adjustments on the 65 tac that I know of. There is a nut on a threaded post on the back, not the + or - elect. which may be the adjustment, never messed with it. Mec. tacs can be more of a problem than elc. ones. Posts on this site say that stock elc. tacs are sometimes off by 400rpm. GM must not of thought that it was important for them to be to accurate. My 65 tac reads correctly for my rear gear ratio and tire dia.

The tach does not run through the resistance wire, it runs at a full 12v. There is an adjustment mechanism on the tach but I've never messed with it so I can't really provide you with info on how to use it.

Thanks, for the info.
Rich-L79 I will do some research on the adjustment you mentioned. If I can figure out how to use the adjustment mechanism does anyone have any ideas how to calibrate it. I can always compare the mechanical tach on the car I tested to my tach and make the necessary adjustments to read the same, but don't now if there might be a more accurate way of calibrating it.

If anyone has more info on this adjustment mechanism on the 65 tach, I would greatly appreciate any help. Thanks, -Kenneth