My old Ford F350 (1974) was seemingly the same way. Three-speed automatic with a 4.11 rear diff, it never seemed to get better than 11 mpg no matter how it was driven either. Unless you count the one time that I hypermiled (unbeknownst to me at the time) it. I was able to go somewhere close to 20 miles on one gallon of gas (albeit under ideal conditions).
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