Sounds like a good setup- I recommend a few round trips at each speed. One thing you'll find is "your mileage may vary" even with seemingly identical temps, traffic conditions, etc, so it may be worthwhile to avergage a little at each speed increment.

Also don't forget that your car is in its breakin period- soon mpg will go up independently of any driving technique, and it will be difficult to calibrate that back out since you're the single test point on this vehicle right now. So running a control is difficult for you at the moment. The HAH and HCH (I think) have generally seen significant mpg kicks at 1200-1500 miles and again around 3k.