syncronizer

When I am running, the tachs are off by about 300 RPMs but the synchronizer shows dead center. Which do i believe and any ideas on how to test this? Fix this? Also the throttle handles are off a little bit and its annoying. Ideas to resolve? Please help! Thank you.

1998 370 DA, replaced original syncronizer gauge, hooked up wires correctly, new gauge does not work. Original gauge works, Easy hook up , 2 signal wires, 1 ground, 1 power, 1 light...hard to screw up. Any feedback appreciated.