My 99 S-10 with a 4 cyl and 5 speed got 27-31mpg. It was a 2wd extra cab short bed with 210,000 miles on it when it was rolled, so a lighter truck should do better on gas.
You can get a real good deal on isuzu pickup engine and tranny's. They were carbed until 93 or 94 ,so you would not have to worry about wiring the fuel injection, they are good engines, have good power, and last forever. They also get real good MPG's