mpg on my 340

Quite a few years ago my 71 Duster 340 was running high 12's with a LD intake and a big Hyd cam. I swapped out the 4.56 chunk for a 3.23 and hit the highway for NY from FLA. I got 18 mpg. 4 speed car. My truck with EFI and overdrive wont even get that.