I agree with the others, if your RPM's are too low on the highway at speed, your mpg's will suffer, especially if there are any slight hills- you'll just have to give it more pedal or put it back into 4th which kinda defeats the purpose.
1968 Camaro - 250 (No, I'm not gonna drop a 350 in it!....Jeez!) 1968 C-10 with 2 extra cylinders