I was thinking about buying a 88 F-150 with an 300 I-6 just wondering what type of fuel mileage to expect. 5 speed, 4 wheel drive.

In general a 300-6 will get pretty good gas mileage. I had a '90 F-150 with a 300-6 and five speed, except mine was 2 wheel drive. It had 163,000 miles on it, and I still managed to get 19 mpg on the highway once. (in the winter too) Overall, it would get anywhere from 12-15 with mixed city, highway driving. Your results will probably be slightly lower because it is 4X4.

Our '83 Bronco 4x4 would get 22 MPG highway with 3.08 gears.

Gearing and speed are everything to these. Gear them slow and run them fast and you'll be lucky to see 16, gear them fast and run the speed limits and 18 is easy.