mid-teens for economy. Actual numbers will vary slightly depending on how the truck is optioned... an extended cab or long bed will reduce economy because of added weight, axle ratio and tire height and width will also factor in. Even if the truck is in good running condition.
Highway 14-18
City 11-15
Driving habits really matter on these older trucks. A buddy of mine had a '92 with the 350ci and 3.73 gears. He could get 18mpg running 70mph.