Here's the results after testing out my new Hypertech tune.
Before programming, and changing plugs I was getting around 17.9 MPG on the highway with cruise set at 75 on the relatively flat terrain between Cleveland, OH and Ann Arbor, MI. At 65 MPH I would get low 18's (18.2-18.5 MPG).
After installing the Hypertech program (and resetting the computer by unplugging the neg battery cable and putting the key in the on position) I started the engine and let it idle for about 5 minutes. I then headed out for a stop and go trip through town for about 10 minutes, then headed for the highway. I set the cruise at 60, reset the MPG and drove for about 10 miles and got 22 MPG. I got off the highway and got back on going the other direction to head home. This time I set the cruise to 70 and averaged 20.2.
Besides the improved MPG, the improved acceleration is really noticable. A side benefit is that my speedometer is now within 2 MPH of what my GPS reads at highway speed.
In hindsight I wish I would have got a good baseline after changing out my spark plugs before the programming so I could tell how much of the improvement was from the plugs and how much was from the program.
Black 2006 XK 4x4, 3.7L:
K&N CAI, RC 2" Lift, 1.5" Precision European Wheel Spacers, Pinch Weld Mod, Falken Rocky Mountain ATS 265/70r17, Hypertech Max Energy Tune