The best tunes for mileage are the hottest tunes you can get with a torque converter lockup, so any race tune using an SCT with custom tunes. They yield the best, but I wouldn't mod for mileage myself. Mod for power, and then if you gain some milage, be happy.
Economy tunes are a waste IMO.
Ruger is right, just take 5mph off your normal speed and you'll see an increase. Try to stay at or under 2k rpms the whole time, it's the sweet spot.