i wanna know more bout slant sixes

well i aint been on here in forever i got a jeep for my first project instead of a duster(dad knew id do donuts either way n he ratherd i do em in the mud lol) and well its awesome but i need better n 13 mpgs i wont be able to drive it thru college i hear a /six is good for 30 mpgs but i worry bout a lack of power i am 17... i would very much like to have me a little pep to it how much could i build it without breakin the bank and loosin gas milage i love dusters i have for a long time i figure i can get one that get good gas milage and keep the jeep for rocks n mud best of both worlds lol