Gas Mileage with my RL6

I have a new RL6 (non-turbo) and I am disappointed with the gas mileage. I have been trying to get good mileage. I drive like there's an eggshell under the gas pedal but I can only get 21.2 average MPG. I expected better mileage out of a 4 cylinder. I noticed that a stoplight the average mileage drops and then goes up when I start moving again. It seems the computer takes into account the zero MPG when idling when calculating MPG. I guess that's more accurate but it makes it seem like I'm driving a gas hog.