Find the cruising speed that gets your AV the best mileage and set the cruise control. a constant hwy speed does get better than on that is foot controled. I would also see what octane level works best in your AV. The highest is not alway the best.
You might want to consider the cost of the upgrade over the cost savings in mileage. There were a couple I have looked at that didn't save me more than I would spend.