Not necessarily. Fuel mileage is primarily determined by throttle position. As you pointed out, you have to give a smaller engine more throttle to match the output of a larger engine at a given speed. Of course other factors like displacement have an effect.
I suggest shooting for a 3000 rpm cruise for best results. Really, anything in the 2500-3500 range should be fine.
4.56 if your area is relatively flat and/or you drive a lot of highway miles.
4.88 if you're in a hilly area and/or do mostly secondary road or city street driving.
