[Richard]

Have a 94 C1500 extended cab shortbed purchased new with 3.42 gears. 350 and a 4L60e. Purchased with the high gears because I wanted to lower 6/8 immediately and put 315/35/17 tires on the back, short tire. In stock form when new and stock averaged about 14-15mpg. 17 on long trips downhill. Still averaged about the same after shorter tires. Installed helper bags and frequently used the truck to haul loads when lowered.

About 8-10 years with that, back to stock height (rear still dropped 2" for level). 275/60/17. Pushing 100k now. averaged 13-14mpg.

Wanted something different after 20+ years and lifted the front 5" and rear 2" over stock. Lift spindles, 5/8" coil spacers in front. Stock height leafs in back with 2" lift blocks. Installed 32" all terrain tires. Milage still at 13-14 but trans hunted in and out of OD quite abit under a slight load.
Rear gears went out a while ago. Posted in my build thread about it. I installed 3.73 as replacements. The OD hunting issue is much better and the truck feels to have a bit more performance. The 94 is my DD to this day and milage is still at 13-14mpg with over 180k now.

So, my take is milage will not change much no matter the gears you are considering. Gear the vehicle for the use (load) and comfort level you desire (RPM/sound level) or use most often.

An add- my 2000 GMT400 based C2500 (350 Vortec/4L80e) with a utility bed loaded with tools, rack and almost 300k averages 13-14mpg.