Rear Gears for gas mileage

in a nut shell your maximum or most efficient lo speed torque must be your cruise rpm
and that is determined by your camshaft.I have seen guys go to overdrive and get worse mileage because they cruz at a very inefficient rpm...gears have the same effect.
there are other factors conspiring against you like the air gap that because it is cooler does not
atomize fuel properly.Big valves,lumpy cam make good power at high rpm but not at cruz speeds and that 727 is a power robber.
..Using vac advance,tuning and operating at 190-195 degrees all can help.