I have the crew cab version of that truck with 4.56's and E4OD, and it gets 8mpg's city and 12 hwy...err...it did when it was running. Keep in mind, they're big heavy trucks...not so much the vehicle you wanna be looking at for fuel economy. Also keep in mind, power adders, extra weight, gearing, trans, etc will alter these numbers.
That being said, your probably going to be looking at 7-10 in city and 12-14 on the highway. figure the lower number if an automatic and the higher if its a ZF. Also the numbers will vary if you are 3.55 3.73 or 4.10
My brother had an 89 F-350, long bed reg cab w/ the famed 460. He got about 9 MPG when he was careful. The worst part was when his fuel gage went below 1/2 tank, at highway speed you could actually watch the gauge drop. He drove it to Florida from Massachusetts, and was CONSTANTLY worried where the next fill up was going to be. (He bought two 5 gallon gas cans half way through the trip as backup....)
Ditto from above. My 89 F-250 HD, 460 with C-6(no overdrive) gets 10 mpg on a GOOD day. It usually runs around 8 or 5-6 pulling an 18' camp trailer. This is one of those trucks where you watch the gas gauge more than anything else on the cluster. The good thing is that I forget the trailer is back there sometimes.
iv got a 96 cc 350 and on a nice cool day i get bout 15-16 on the hwy and about 11 or 12 in town...and the wind will be your enemy...when i drove from wichita falls tx to houston tx (about 6 hrs.) i did about 75 the whole way and got about 12 mpg but the strong head wind didnt help much...and the fact i was doing 75 didnt either