Impressive, Obermd!!! Did you ever get a chance to do a check from Eisenhower to the twin tunnels?It was but it wasn't all downhill. That run was from the Shell station in Fairplay to the Conoco in Bailey. While a lot of downhill there are two passes (Red Mountain Hill and Kenosha Pass) between them I was above 50 MPG at the top of Kenosha which is about the mid point on that segment. That's why I also posted that I've seen ~60 MPG on "flat" ground for 25 miles.
About a year and a half ago I drove from my home on the south side of Denver to the University of Wyoming in Larimie, WY and back. I took the same route in both directions which eliminated elevation changes in the fuel economy calculations and still ended up on the high side of 55 MPG for the day.
The ECO MT really, really likes altitude and hills. I get 1-3 MPG improvement in overall fuel economy in Colorado than I do when driving the car in the east.