318 fuel mileage

I was deffinatly going to go with HEI upgrade I hear it makes a huge difference, and of course a huge tune up all the way around. I want to say when I was running the old 318 and didnt know at all how to tune a carb or timing it was getting around 16mpg and thats in a fairly light weight 71 2door swinger. I'm still kinda partial to the little 225 and it runs ok I was just considering putting the 318 on a stand and rebuilding it for the experience and when I can afford the gas maybe drop it in and then do the same to the 225.Thanks for all the info people, keep it coming if you would like to add more.