What did the 84 F150's get for milage? 302, 351 (2/4barrel) Automatics?

10 to 16.

Depends on the engine and your driving style.

Your joking right?????????????????:shaking:
Seriously, gas mileage can be affected by so many things, operator driving habits, engine efficiency or lack thereof, wind resistance, dragging brakes, tire air pressure, how much added weight is in the back of the truck due to garbage, tools ect.

Well will a 351 get better milage when driven conservativly since it doesn't have to work as hard as the 302 to move the truck?

possibly, if all other factors are equal. But either way, between 10 and 20 is where your going to be sitting, i'd say in the 14-15 range with either one, being that it has a slushbox.

Im guessing a slushbox is overdrive?

http://www.fueleconomy.gov/feg/sbs.htm

this webiste will let you know what type of mileage you will get with 2wd, 4wd, manual, automatic, 302, 351, etc.