Tale of the faulty fuel gauge!!!

Okay guys, I'm really stumped here. I cannot get the fuel gauge in the dash of my very stock 74 Duster to work right. When I fill the tank completely, it only shows about 7/8 full on the gauge. After driving for 40 miles, it shows half a tank remaining. The sending unit is new, as is the tank. The gauge cluster was rebuilt by a very good gauge guy, and it has a solid state voltage limiter. I've tried a different sending unit - no help. The new one does the same thing. I've had the original tank sending unit rebuilt, and it does the same thing too. I've replaced the ground strap on the tank with a new one. Grounding the sending unit wire to a body ground with the ignition on sends the dash fuel gauge to somewhat past full. Any ideas?