ok i got a 1994 Chevy silverado 5.7l v-8 4 barrel carb not towing anything no camper or any thing diving habits are constant i need to know in easy terms what is over drive compared to drive dose it save gas? what is my mpg? and i also have maintanace that needs to be done it is the following

complete clean of the engine (inside)

new belt,filters, fluid, hoses

rust in the cooling system

interior work (seats, headliner, remove mildew, dash, glove box door fell off,new replacement sidepannel for the door, carpet stain removal)
new battery and terminals also electrical system check
transmission totally cleaned completely (inside)
body work (dent the size of a watermelon right by the gas cap on the bed, a little rust here and there, new grille little dents here and there, door clips fender when i open it dosent sling open till it hits the fender if thats what your thinking, the inside of the driver door isnt hooked up locks, handles etc.. rear gate handle dosent work and i need beddin
brakes need to be bled and i have a new radio i need to put in and rhino lining

overdrive will save you fuel on the highway, and sometimes around town as long as you arent climbing any grades, your overdrive usually wont kick in below 50mph sounds like a lot of work, I do a little work on the side, but then it depends on where you live a little at a time