Both the Street and Performance tunes are built with economy in mind. With that being said, it's all in your driving habits. Many people get the best mileage with the Performance tune because it lacks the low-end torque of the street tune, which makes you want to use it more. Many people experience just as many MPGs with our Race tune as they do with our Performance or Street tunes, because of they're driving style.
Load up any of them, and experiment. You'll find the one that gets the best MPG's for you!