Hello all,

I completed an 8.1L swap last year in a 70’s GM truck using a MEFI-4 ECM. Never got that damn thing tuned right so I just replaced the MEFI-4 system with a P59 PCM (basically a stock Silverado PCM with a custom tune) using a Howell Engine Development 8.1L harness. The truck runs 100% better but my tachometer now reads like half what it should.

I did a search and found a few hits but most seem to be that the tach reads high until an adapter is used or use a 4 cyl tach. I noticed those threads were a few years old. My tach reads low.

Anybody have any insight how to make this tach read properly or what tach should be purchased? I am currently using an older Sunpro mini tach. I don't have a problem getting a new tach, just not sure what I need.

Thank you