Since the taller tires actually reduce the gear ratio, like going from a 3.23 to a 2.94, it can hurt mileage in stop-n-go traffic because the engine torque isn't working as efficiently with less overall gear. You may also be pushing down on the pedal more to get it going like it use to. The taller tires also make the car sit higher, hurting aerodynamics, but that may be insignificant at lower speeds.
But the lower speedometer/odometer readings may make the most difference in your figuring. Your odometer would read LESS mileage because the taller tires would make less rotations driving the same distance, so the odometer would reflect fewer miles.
