Determining RPM? 1998 Johnson 9.9 4 Stroke

[J2Boat]

I have two different gauges that have tachs in them but they are for 12 volt systems and I cann't figure out how they can be hooked up if at all. Is there a gauge that can be used and how would it be hooked up?

Thanks

 

[ibnfshn]

Check out Tiny Tachs. They get their signal from a spark plug and work good.