IIRC you can get good gas milage with a 4.36 because the engine dosnt have to work as hard to move the car since the gearing is taller. It does put you at higher rev's but its negated by the less work the engine has to perform. I'm sure if you went to a 4.6 your gas milage wouldnt be as great though.

Its the same as if you went to a 3.54 on a low powered car. Your rev's would be super low but it makes the motor work alot harder to rotate the drivetrain.