Over the years on my trucks I have had several combos---with the Q jet on my 84 chev 454 about the best we could do on milage was 6 mpg---On Fords 460 with the 780 vacume Holleys around 9 mph and with the 850 double pumpers could get 11-12---all this was when pulling 5 wheel trailers on long road trips--the 850 dp worked best because we had lot more available BEFORE the seconaries cracked open