I saw this on another forum, written from someone at American Auto Wire who make repro harnesses for our old cars-
"Most Original OE analog tachs WILL NOT work with an HEI distributor, period. You need a tach filter in series between the tach wire and the distributor. We field at least 10 calls like this every month. Occasionally, and I mean occasionally, we hear about this one or that one working, but more often than not, they simply do not work at all, or read way high or way low (I have never had one work personally). The waves that the points emit are different then the pulse that the HEI puts out. That is why EVERY single big cap style HEI equipped Chevy car had a tach filter on them (other GM's also. My 1977 Pontiac Can Am had one as well). Be it Corvette, Chevelle, Nova, or Camaro (Heck, even the VEGA and Chevette), they all used them from the factory. I just went thru this with a 70 Monte Carlo and a 1966 BB Impala. Both were new harness installs with factory in-dash tachs and both were using a big cap HEI distributor. Neither would not work at all. Put an NOS 74-79 style tach filter in series between the tach and the brown lead on both cars, and viola, the tachs worked perfectly!"