at 60mph the engine is turning about 2150rpm. if you consider the amount of drag created by the vehicle and the energy needed to keep it rolling, a taller tire could actually reduce mpg by not allowing the engine to make enough torque efficiently. I've got my eyes on a set of continental tires that are LRR and allow for 50psi.
Continental*CrossContact LX20 with EcoPlus Technology
the only down side is they are suv tires and weigh 32lbs each.