How fast are you driving it. If you are going a little too fast you're on the fine line of the power valve in the carb and it's dumping fuel. Slow down about 5-10 mph for a week and see if there's a major difference in mpg. If so, a gear change will fix the problem. You may have a bad power valve also,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!