Are you measuring this with the car's average readout? Do you zero it out before you hit the highway or when you start the car? I use a scangauge that zeros every time I start the car so it factors in idling for the trip before you begin, and it can also tell mpg for the tank. Great little toy.
Areas where you go 50-60mph will have more stoplights, traffic, and fewer lanes -requiring slowdowns for turning vehicles - think country road. I get my best mpg at 40mph because my car goes into 5th at 36mph and the T/C locks at 39-40mph so I'm driving the wheels with less loss of power transfer and I'm not pushing much air out of the way of the car (vs at 100 you're doing a lot of that).
Warm days with low humidity will also give you a good mpg bump, my car hates rain.
Also if you get a Scangauge you can watch the gallons/hr readout and it's fun to see it read zero when you're coasting at speed. I get 36mpg on trips around town with low average speeds if not foiled by red lights and buses because I can coast down hills for nearly free.
Correct, the average read out is what I use. I find the real time (after every 2 seconds) read out, pretty worthless. It is all over the place even when I am using cruise control. I have zeroed-it-out only after my fill ups. My mpg of 34 mpg was write after a fill up. Unfortunetly, I live in the Chicago area which at this time, of year is humid. My treck yesterday was is a partial drizzle/rain but was warm at 78 degrees. The average mpg gage does a good job of factoring in idling and stopping. Your toy sounds a little more sophisiticated and probably more fun. This car is so new (1 week), I will give more updates if the mpg increases or stays the same. Thanks for the info!