I just posted this in another thread.
The biggest thing to getting decent mileage is smooth driving. Try to predict traffic patterns that will allow you to avoid the brakes as much as possible. Doing so also keeps you more focused on other folks driving helping to avoid mishaps.
I didn't get the Z to be a gas mileage winner, but I do want it to be reasonable to drive to work. I'm happy with 23. If you're getting 27 that's fantastic as my old Accord 4 banger got about that.
Now one thing that blows my mind. I can drive my 600RWHP Challenger to the drag strip, race all day running in the 11s, and drive it back home and average nearly 24MPH in it. Eat your heart out EPA.