Technical Electric fuel gauge jumping around. Fixed. Update in OP

Remember one important fact about the 3 terminal regulators is that the input to output voltage difference needs to be usually 1.5 to 2 VOLTS. This is with the 7800 series regulators. So, if you are trying to have 12 volts for your gauges you will need somewhere around 13.5 volts input. If you want 6 volts for the gauges, then you should be fine. BUT you should ground the tab of the regulator to a good metal ground for proper heat transfer. And remember that the gauges will vary as far as current draw from full to empty. Depending on what the gauge shows when the sender is near ground will be the highest current draw, and should not exceed the 3/4 amp capacity of the regulator.