Carb Help Needed

My Holley 750 DP got 8 mpg on the highway, and ran 12.26 best. My Thermoquad gets 14 mpg on the highway, and has run a best of 12.09. (same 408 through auto with 3.73's) I have heard many say that Holleys are bad for mpg, and i could not get it any better. But then I never learned how to tune it, either.