1966 Mustang Coupe, 5.0 Swap, EFI/MAF, T5z

Trying to get the fuel cage calibrated in my ride.  I did a switch to the Dakota digital gauges.  The custom calibration calls for starting w/ an empty tank filling it 1/3, tanking a reading, 2/3 take another reading, fill it and take final reading.  Sounds easy and in my first effort I let the car run out of gas and it only took just under 15 gallons which through the reading off.  Second time I did it the tank filled with 14.5 gallons which messed up the calibration again.  This blew my mind b/c a standard tank is 16 gallons.

In part 2 of my adventure I let the car get low on gas and then drained it.  What I discovered it that the fuel tank spigot is actually about an inch off the bottom of the tank, in a corner, and the tank slopes back naturally.  So I actually jacked up the rear of the car about 2 feet on my level driveway and drained it completely.  I then measured out 2 gallons of gas exactly, and went to the gas station.  I did in fact top off at 14 gallons making it a complete 16 gallons.  So that explains that... so for future reference, a 65-66 gas tank will have about 1-2 gallons of fuel in the tank when it runs empty just b/c of the geometry of the tank.

I went ahead and pulled the fuel sender to get the resistance readings.  It came out to 85 ohms/ 15 ohms for empty and full.  The problem I'm having now is that the gauge is displaying empty even with a full tank and the resistance test is saying "000 Ohms".  I'm not sure what that means.  If it was a bad ground, the manual says it will display "999 ohms".  I read on another forum that there is a tab or something that needs to be properly connected or ground.  Idk, Dakota Digital isn't open till Monday.  I ordered 3 - 5 gallon gas jugs to do the calibration at home.  I don't think the fuel sending unit could be damaged but it's possible.