I just swapped in a 2.8 for my duke. Everything is good except now when I'm cruising at 70 mph the tach reads 3400-3500. I"m still using the isuzu 5sp. When I had the duke in I was cruising 70 mph at approx. 2400 rpm. I changed the tach gears in the tranny to see if it made a difrerence but the gears were both red and identical. No change in tach rpm. If using the same tranny, shouldn't I be at the same rpm at 70mph? Can someone explain?

The gears in the tranny are for the speedo not the tach. But think about it - you're running a 4 cyl tach on a 6 cyl motor. you will have to get a 6 cyl tach

The tachometer measures engine RPM, the speedometer measures wheel RPM (MPH) the tranny sensor is the speed sensor, the tach feeds of the ignition module, you will need to modify or replace the gauge

Thanks all, I have a v6 speedo and tach cluster. Didn't even think of that. Replacing it now. Thanks again.