Horsepower with mileage options

Yeah the truck is diesel. Freeway driving is minimal with the valiant. What gear ratio would be an even trade off of get up and go vs Mileage? As I'm sure with most of us on this forum, I've never built an engine for mileage. It's why I daily drive a Kia soul and leave the hot rods for car shows and drag strips mostly. But my son from the first time he drove our old 68 fury has been in love with classic muscle cars and hair bands. Lol. My boys drive our cars when their tanks are low in fuel, which is more than they like. Hence the reason he started thinking about getting more mileage out of the valiant. He lives to drive that thing, which he does to school daily, but wants to be able to drive it to go out with friends and what not without having to put more fuel in it. It's the new generation thing, style and mileage, he just doesn't want to drive a ricer if he can get the mpg he is looking for in a classic.