Welcome to the forum! Sweet looking XK. Lower valance? If your talking about the the plastic air dam\splitter on the bottom of the front bumper that will easily cost you 1MPG just so you know. Getting 10MPG is kinda low for a stock XK. These things are awesome but really like their maintenance a lot. I just bought my second XK not that long ago and the first thing I did was replace all the fluids (went from 10 to 13MPG) it will help. As soon as it warms up its getting all new plugs, PCV, water pump, and front hubs which should get me a couple more MPG's. Also look into the OME LIFT.
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