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unsure what year youre dealing with, but i can tell you this to be true on a 1968 - if it has a rally pac im assuming its a 68-72 and this should still apply, but unless its 1968, i cant promise it will.
im unsure by reading what you are trying to accomplish with the gas part of your question
the gauges on a 1968 are grounded by thin little metal straps on the dash. The 2 screws that hold each gauge in the dash pass through the metal straps to ground that gauge out. Between each gauge is a grounding jumper wire that runs from gauge to gauge. The fuel gauge should have a ground strap and the tach should have a ground strap, but between the tach to the speedo, and the speedo to the cluster you will have jumper wires that bond each gauge to each other (at least mine did). if all 3 gauges arent bonded to each other and to the ground straps, make it so.
at your gas tank you will have a ground wire that grounds the tank to the chassis sheet metal. crawl under your gas tank and just in front of the gas tank on the drivers side you should see a ground wire running up to a screw that screws into the sheet metal - make certain thats a clean good ground.
grounding the brown fuel gauge wire (not the wire that runs to your tank, but the one that runs through your intermediate wire harness to the dash) should give you a 100% empty reading on the gas gauge - if you are curious about testing the wire that runs from the tank to the gauge, pop your trunk and find the brown clip that goes through the trunk and down to the gas tank behind the liscense plate. Unplug it and ground it out to bare metal somewhere (ground the wire that runs to your dash, not the one that goes to the tank). Turn your key on and the gauge should move to the 100% empty position. If it moves to the 100% empty position then the wire running from the gauge to the trunk is good, and the fuel gauge is good, your problem is either in the wire from the trunk to the sending unit, or the sending unit. If it doesnt read 100% empty then either your wire from the trunk is bad or hooked up wrong, or your fuel gauge is bad.
the sending unit is a Rheostat - when the float moves it gives you different levels of resistance which is what your fuel gauge reads. if you pull the sending unit out of the gas tank and hook up an OHM meter from the 2 terminals, when its full it should read close to 90 OHMS of resistance on the Ohm meter. When the float is moved to the empty position it should read close to zero OHMs (the equivalent of grounding out the wire in the trunk or zero ohms of resistance)
of course a half of a tank should read 45 ohms resistance on a 90 ohm sender
that should troubleshoot your fuel gauge
your tach hookup - you should have 3 large binding posts that form a triangle. There should be black insulators on the top post, and there should be 2 binding posts with red insulators on it.
Looking at the gauge from the back, the red binding post on the left should be your clock hookup
the red binding post on the right should be the brown wire that has the 2 amp fuse holder on it and the bare wire protective covering over it and this runs to the negative side of your coil.
if memory serves me correctly the top center binding post hooks up to your dash lights - directly above that will be a sticker - peel the sticker away and the small screw inside of the case is how you adjust the RPM reading of the tach
hope this helps
Mike
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