A good portion is the gears and the engine setup. My stock '78 Bonneville with a 400 and 2.41s would get 19-20 on the highway and 13-14 in town while my '75 Trans Am with a 400, Melling SPC-7 cam, and 2.56 gears gave me 19MPG when I drove it to San Diego this past summer. My '72 350 2bbl Lemans with a 2.78 rear end never gave me any better than 14.5 in town and 17 on the freeway.
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