My tach and gas guage are confused?

[rallye5.7]

Yesterday was the first day that the car was legal (insured and licenced) for the road and I knew that the tach was lazy and fuel guage never showed more than 1/8. As I drove around a bit I noticed if the tach quit working the gas guage would slowly go to full (the tank was full)and if the tach started working gas guage went down to empty. This may be a coincidence but could they possibly be related??

[Aussie Challenger]

  Check the ground wire for the instrument panel, make a new one if needed, I did and brought everthing back into range.   

[MEK-Dangerfield]

Those two gauges have different power sources, so the only common link here is "ground", as was mentioned. The gauge cluster needs to be firmly screwed into the dash frame, and you might want to check that the ground strap for your gas gauge sending unit is solidly in place. On the pick-up tube of the sending unit, there will be a steel strap that runs over to your solid fuel line. That is where the sending unit gets it's ground.


  Mike