Thanks for the positive feedback guys. I haven't been driving it much since I have no classes during the summer. Fall term starts on august 26th so that's when I would really know how installing the air dam effected my mpg's. I've added 93 octane when my XJ had roughly half a tank of 87 octane so that equates to 89 octane midgrade fuel. I immediately noticed smoother idle, especially when the engine is cold right after initial start-up. The gas level looks to be dropping slower too.
Its may improve a bit with the xj not being lifted on big tires this blocks a lot of the air that hits the axle steering and other components down low. So it should do something as for how much guess you would need a before and after to get a real test.
any air directed around or over instead of under on an XJ is going to be a gain in aero and MPG as a result, however slight it might be. Dont particularly like the look myself, but certainly a good idea for a DD