this may not be an apples to apples comparison. i have two cars, b-bodys, both with 440 6-paks, automatics, 3.23 rears. one car is a 69 r/t 272-.455 mopar came, stock exhaust and converter; it'll get around 15mpg on the highway. the other has an engle solid cam (238@.050-.510 lift), street hemi converter, big tti exhaust; it'll do a little over 14mpg. i run vacuum advance distributors and tailor them for driviability. i think the small center carb and vacuum distributors are a big help for mpg and also kept the engine cleaner. big carbs, single plane intakes and full centrifugal distributors wittle away at mpg.