My experience with various cars is that the RPMS don't drop until you come to a stop...not sure why, but that just seems to be how the IACV works.
If you put 20k a year on the car, getting a car that gets 50 MPG over a car that gets 30 MPG will save you 800 bucks given gas at 3.00, and it's likely to be higher than that, so it might really be worth looking at!