289 with Duraspark and other stuff
 

Folks, I have a '65 289 (289 block/heads) with the crank, rods, and pistons from a late 80's 5.0 HO. It's also running a 1981 Duraspark distributor and ingition module, but with a TFI coil from a '95 Bronco. I bought it like this..not my work! Anyway...after hooking up a new Auto Meter tachometer EXACTLY as the instructions state (and according to 2 calls to Auto Meter tech support), the tach still would only show just about half of the actual engine rpm. Could this "mix" of igntion components be at fault? Also, have read that those TFI coils aren't too reliable, and tend to catch fire. Any opinions/experience on this odd-ball ignition system combination?

Try switching the tach to 4 cyl mode to double your signal output?