You don't use a "set amount" of gas at any RPM.It takes more gas to keep 1600 RPM @ 70 MPH then it does @ 30 MPH. Aerodynamics in our cars aren't too bad, we're very low, our wipers are hidden (some of us :P ) etc

Today I put the CC on and hit the decel button to see how low I could go. I dont have a DIC, but I think 50 is about right, lower than that and the RPMs dont really go down a whole lot.

Sometimes my torque converter locks up @ 45mph and the rpms go so low that the car starts jerking, it doesn't like to disengage once it has engaged either.