problem with defi rpm gauge

[Ivan141]

You can build your own tach signal off of the coil- signal, but I suspect the DEFI gauge does that itself already.

Best way to do that would be to use an optocoupler with a pullup resistor to +12 and make whatever signal you want on the other side. This is a commonly used input circuit on megasquirt tach inputs.

What I would suggest though, is that you have a good look at your DEFI tach and make sure it is built/configured for 4 cylinder operation.
I very very much doubt it is reading the signal wrong, I just think it is expecting a 5th 'bang' for every rev, which would explain the 1.2 offset you are describing.