In my 62 Belvedere .... stock and worn-out 440 with a 509 cam.... TQ with a SD intake and iron exh mans and a 2.76 gear ... got 17 mpg ....

....and ran bottom 12's ...at 100 mph or so.(BAD speed for that ET because of poor ring seal).

Something REALLY is wrong with your FRESH combo. I would check the primary jetting on that carb. It has to be way WAY fat.

Suggestions? .... a Plastic Fantastic ...aka ThermoQuad.