My 5 sp seems to worth about 10 MPH over (my father's '74 240 4sp) 4th, so you'll be able to cruise at the same speed with that many fewer revs. As I don't have a tach, I haven't a clue what the actual spread is. It's a great car for the flats of Florida, but doesn't have a lot of grunt in 5th and slows a bit on overpasses, even ... Delivers between 31 & 34 average MPG, tho, and will cruise around 80 without apparent strain -- 77-79 is my usual.

Ed