building a long distance touring car

I few years back I took my 340 Duster on a 2700 mile round trip from Fla. to N.Y. state. it was running 12.80's at the time., 4 spd car. The only thing I did was swap in a 3.23 gear. The dang thing got 18 mpg. My newer Chevy pickup with OD and FI cant hit that number. The old stuff can work.