Don't buy into the big cubes = better miledge bologne. There is a whole lot more to it than all that. You want me to throw out some numbers, my 318 I had in my cuda got a best of 29 mpg the 360 in it now barely will get 20 mpg. If bigger motors got better MPG then a prius woul have a 512 hemi under the hood. If you take two similar engine lie say a 3.9 v6 magnum and a 5.2 magnum and put them in the same car the V6 will get better MPG UNLESS you force it to accelerate the vehicle as fast as the 5.2 did, for exaple running 75% of the engine power when you are trying to merge instead of 50% of the 5.2 power, if you push it to get the same performance it will use the same fuel, the 5.2 has the potential however to use 25% more fuel at WOT than the V6, people just don't push them that hard and so every one claims the V6 uses as much fuel as the V8 with is only true if you expect them to perform the same, drive the V6 like a V6 instead of trying to demand V8 performance than it will get better MPG. I have owned plenty of both vehicles and my V6 dakota gets better miledge than any of my V8s ever dreamed unless I am hard on it. If you force it to down shift more it revs more and you lose the MPG improvement.
Higher throttle angle means less vacume in the intake resisting down ward movement of the piston and costing you miledge. That is one reason why higher gears help MPG. Higher RPM also makes more firction.
It only takes mabey 20 hp to maintain reasonable highway speed in an average car, if your motor is capable of makeing substantially more power at your cruising RPM you are wastin energy. The reason car makers do not set cars up to run that low RPM is because then you have to down shift to go up a hill and people don't like there car to shift, that is why we have MDS and dual VVT (both technologys make the motor more efficent by reduceing intake vaccume at cruise) motors now combined with higher gears, the new motors make more low RPM tq so they don't have to shift as offten with the higher final drive ratios.
In my experimenting high compression and quench make better MPG along with less overlap and high flowing small ports, then gear the heck out of it and watch your cruise MPG go way up. If you get the compression too high retarding your cam can reduce intake vaccume, reduce cylinder preasure, give you combustion more time to press on the piston and you can get even more MPG, in it's exagerated form this is called Atkinson cycle, another technology used in modern cars for increased MPG.
