Re: what gauge would i use and how to intall it?
Engine RPM (revolutions per minute) are measured with a Tachometer. Tachometers are generic so any brand will work so long as it can be calibrated for 6 pulses/revolution which your Johnson engine provides. The tachometer (called a tach) requires +12 volts when the key is on, ground, and a signal (pulse line) from the engine. If you look at your control box just below the key you will find a three pin connector that provides these connections. You can obtain the correct wire harness when you order the tach, get it here on iBoats, or even Cabelas carries them. As for speed of the boat, just like your car you need a speedometer. It requires no electrical connections except for the internal light if you run at night. The speedometer is generally supplied with a flexible tube that connects to the back of the speedometer and is routed to the back of the boat where you attach the pickup which is called a pitot.
