I do think you need to try another meter. Numbers you're posting are not matching up with logic or what they should be.
Sending unit rheostat range is probably going to be 0-30 ohms or 0-35 ohms. Have seen both listed for sale for the "64" model...along with 0-90 and 10-90. The ones that go up to 90 are listed as fitting "64" to "67". The 30 and 35 are "64" only.
Numbers:
.8 VDC in the rear we'll come back to.
11.8 VDC going to the guage is what's to be expected.
Now then, the guage has three connection points: power supply (B+), ground (B-), and the one that goes to the sending unit.
2.8VDC??? what could that be
We've got two points on the guage unaccounted for so far, ground and sending unit wire. Ground would ohm out at 0/zero ohms (ground to ground = 0) and sending unit wire. Now 2.8VDC sounds alright for the sending unit wire, BUT, it checked .8VDC at the back of the car.
So, numbers are not adding up. The voltage drop from front to rear, if it is the 2.8VDC, would definitely be a problem. But you stated that the guage wouldn't drop back to empty even when grounded up front. This sounds like two problems.
Try another meter and retest things. Besides the B+ on the back of the guage there is stud that has a voltage regulator (flat, wire wound, resistor) between it and B+.
Need the B+ reading, opposite end of regulator reading, and sending unit out reading. Sending unit coming out of the guage may be the same as regulator drop reading. And the sending unit out of the guage reading should be the same at the back of the car .
Enough for now
Clay