1994 Chevy Cheyenne K1500 throttle and stuff

[Jeep_Comanche17]

hey yall bought a truck 2 weeks ago and just trying to figure some stuff out. first off the throttle is stiff. it will be idling and ill tap the gas pedal and it will instantly jump 300-400 rpm's. dont get me wrong its fun as hell to drive like that but i only get 12 MPG but it has a 350 and 4:10 gears so that dosent help but id like to get at least 14 MPG lol. if anybody knows how to fix that stiffness issue it would be greatly appreciated!

[00Silverado4x4]

Are those stock size tires? If not, this will throw off your mileage as the odometer will register less miles then actually driven. Also I don't think the 4.10 gears were stock either which will also throw a wrench into the mix of things.

[Jeep_Comanche17]

the previous owner installed the 4:10 gears and the Eaton limited slip diffs. i cant change those or the tires as my work takes me off-road and i have to haul a trailer sometimes too. and no they are not stock size but they are not much bigger.