As other have stated, it depends on your motor alot and the type of driving. I do not know how scientific this is, but I was told you'll get your best MPG when you run at your engine's peak torque RPM. So if you have a turbo 350 and say your engine's peak rpm is around 1800 ( I think that's about where a stock SB is) and that's the rpm you run at 65, then you switch to a 700r and the same driving drops your RPM down to say 1500, it is possible you'll maybe see a drop in your MPG.
It seems to have always worked for me when I had engines built and set up a car that if I got my highway RPM's in that peak torque range, I got the best mileage out of the combo. If I mess with tranny or rear gears that would change where that peak torque was, then I would lose MPG.
Again I don't remember what it was based on, my Grandpa showed me that when I was around 13/14 (long long time ago in a land far far to the north
)and it always has seemed to work out.