Last weekend was our 4 day trip to Lincoln, Nebraska from Colorado Springs (540 miles) and back. Here are our data points.
For the 1st time ever towing, got over 22mpg for the first tank of fuel (Colorado Springs to North Platte). The 2nd tank (North Platte to Lincoln) was 20.2 mpg.
On the return, I saw 14.8 mpg from Lincoln to North Platte, 15.1 mpg North Platte to just east of Denver, and 19.2 around Denver and home to Colorado Springs.
Speeds were a pretty consistent 65-67 MPH on interstate highway for all but a few miles. Normal mileage for our minivan (not towing) is 22-23 mpg averaging 5K-6Kft altitude. In Nebrasksa, normal mileage is 21-22 mpg.
Based on the experiences of this trip and others, I have concluded that the factors affecting our fuel economy are (in this order):
1) wind - by far the biggest effect, can change mileage by 40%.
2) altitude - I seem to see 10% improvement for each 5K ft of altitude
3) outside air temp - warmer is better, even with air conditioning, but just enough to notice
On the way to Nebraska, I had no wind to 10 MPH tail winds with temps rising from about 50 degrees (we left at 5AM) to close to 70. On the way home, we started with 25-35 MPH NW (close to head winds), that decreased to about 10-15 MPH as we approached Denver. Temps peaked at 77 degrees at Denver. Winds from Denver south to Colorado Springs were on our tail, but it's a climb from 5,200 ft to 7300 ft in 50 miles.
I'm glad I'm not towing a high wall - I believe the extra frontage area would make a noticeable difference to our 6 cylinder minivan. And I can barely see over the standard height out the rear view mirror.
2014 Rockwood A122 - tow weight averages 2800lbs
2008 Hyundai Entourage - minivan, 3.5L V6, 250 HP, 5 speed transmission