My 1996 C1500 with a V6 and 5 speed had 25MPG as a average.
My 2000 Silverado 4.8l V8, 5speed 4x4 extended cab averaged over 21 mpg when I owned it.
MY moms 2002 Silverado (same as above but a auto) averages 20 mpg.
The last small truck I had was a 1982 SR5 toyota and it got in the low 20s. Its still getting that with over 250k on the clock, but no way is it anything like the Silverados or C1500 for use as a truck.
Take care, bill