I made a terrible graph but it kind of explains the point.
If you're only improving aero your engine RPM's will be exactly the same, but your load will be lower. Unless you're already driving at full throttle to maintain your speed, chances are you will move away from higher brake specific fuel consumption efficiency. In other words, you will use less fuel, but your engine will become slightly less efficient because internal combustion engines in general are more efficient at higher loads, at least up until they start enriching the AFR.
Ideally, you want to lower aerodynamic drag and rolling resistance as much as possible and then measure how much power you really need to maintain your desired speed on your desired road. Then size the engine so the amout of power needed can be placed as close to max BSFC efficiency (which would be a very small engine) all the while making your engine big enough so it's not going to dog out when you do need the power. The more varied your driving conditions (e.g. hills, etc.) the bigger the engine you'll want. Then also gear the transmission to put those RPMs as close to best BSFC efficiency.
