[rbohm]

the biggest single factor is cam selection. too large or too small and fuel economy suffers. when you build an engine you need to first figure the rpm range the engine will spend 80% of its time in. you then pick parts that compliment each other. after that it is a matter of tuning the combination. one example, my old 289 in my mustang. on the road it would get 25mpg, and int he city it would get 17mpg. i actually had the wrong cam in it but it wasnt too large for what i wanted. if i used a cam with less overlap and more lift i would have gotten better mileage.