What's the absolute maximum range of your Duster?

why does your /6 Duster only get 14.7mpg? Rather than screwing around with the gas tank, I think I'd spend time getting more miles out of the 15.1 gallons...

What gears are in the car? what year automatic? lock-up converter?

I'm building a car for long trips and was surprised to find out the Chrysler changed the 904 gear ratios in the 80s with lower gears in 1st and 2nd meaning you could use higher (numerically lower) rear gears and still have the same effective ratio in 1st gear as an earlier 904. Add an electric lock-up converter for higher speeds activated by a simple switch in the car and you could do better than you are. But even with the early 904 and no lock-up you should be bumping 20 mpg.