I 'm guessing one uses mechanical to get readings whereas the other uses electrical signals to get them.
The reason I'm asking is becasue i have a new autogauge water temp gauge ( electrical) and I noticed you have to send a wire to the ignition, is this so it can get power to work?
also- where the hell is the ignition wire?

 

Don't you have a manual?

Mechanical will use fluids for the measurement. IE you will have an oil line to the gauge for pressure or fuel line for the same. Bad idea running those things to the pass. compartment.

Electrical will use a sending unit that's tapped into a line or housing in the engine bay and send its signal to the gauge.

The wire you're talking about is for power to the unit when the key it turned. Just find a switched source on the fuse block (gets power after key is turned) and tap it into that.

 

Plus on the temp gauges (mech) the fluid cools slightly before the gauge has took the reading. So you don't get an acurate reading from them. Unless you mount them on you bonnet (hood) lol. Also don't want to sound as if i am taking the p*ss but if you put a little threat lock on the sender wire it will not come lose after a while and give you a false reading.