If you drive the car on the highway at about 60mph, and you get an aftermarket CAI with a tune, mileage goes up. Turn the airconditioner off, and mileage goes way up (in my limited experience).
I have seen 29 mpg highway driving under the right conditions (level, slow, no A/C, no wind, and the car is in a good mood).
Believe it or not, I was looking at cars like the Honda Insight, before I bought my Mustang. I drive a lot of highway miles, and the thought of high MPG was very attractive. I bought the Mustang, because I didn't want to drive a $28,000 tin can all day, and I have never regretted that decision. Keeping it under 60 mph is impossible, though- I love this car so much that I find myself spending most of my time trying to exercise what little self control I have left. 100 mph in my Mustang feels like 50 mph in my Corolla!
Thinking about setting my speed limiter at 80 mph!!!!
Good luck to you.