There is a thread on mileage where you can check out peoples numbers. While a taller tire CAN give you better mileage you have to take into effect the taller tire takes more power to turn, possibly dropping your mileage.
As for lifting your car, the taller it is the more wind resistance it hits and the worse mileage you will get. With an average speed of 55, driving mostly highway miles, I am happy to get 27mpg.