I wanted to share a success story with a cheap and easy mod I've made to my own truck but many customers as well too.
If you have nearly any year S-series truck, especially the 4.3 models you can easily pick up a few HP and about 1/2 mile per gallon but ditching that heavy fan.
Although stock it's a thermal design which supposedly disengages after a while it still drags on the motor in a big way.
I bought a $35.00 adjustable electric fan control kit from Summit Racing and a junkyard Camaro/Firebird V8 electric fan setup. Took me about two hrs to install it, removing the factory shroud is a real PITA.
I immediately felt an increase in throttle response, it got rid of that "driving a windmill" sound it makes and picked up another 1/2 mile city. I don't do much highway with the truck so sorry no figures there. Not a big gain but a gain non the less.
I cant see why this wouldn't work on other vehicles.