i had a 90 sierra 4.3 with cruise control and now a 99 silverado 4.3 without cruise control ... both manual 5spd ext cab open beds ... 19mpg is the best i get on long trips at 70-75mph, but i still get 19 even with 160k miles, with it unwashed ... drag is proportional to the square of the velocity, the extra 5 or 10 mph and body grime is costly.

 

Key Word... Underpowered.