It's my understanding that there are only two controls for the fuel/air mixture. The first is the cold start valve and the second is the air mass sensor. The air mass sensor (4" hinged "plate" under the air cleaner) some how mechanically controls the opening size of the fuel ports in the fuel distributor. You might also have fuel delivery presssures which are too high. I cannot imagine how rich the mixture must be to only get 7mpg!

Another explanation might be that your transmission is not shifting out of lower gears, but you didn't say what your revs were at 70mph.