I've made nearly that same drive, twice.... out and back! :laughing: My guess is they took 74 out to Davenport, IA, to 80 to 29 or so and somehow ended up on the eastern edge of SD in Sioux Falls, then took 90 all the way across SD to WY and MT to Bozeman. I'll tell you this... the winds in South Dakota are brutal. In Mitchel, SD on November 11 the wind was 13MPH and on November 12 it was 22MPH. With gusts to 44. Depending on which days they hit, that could have been the issue. Also, historically, they would have been going into the wind. And the speed limit is 80 out there... so sure they were legal.... probably going 80-85 mph into the wind towing a box trailer. I wouldn't expect any truck to get anywhere near "advertised" MPG under those conditions.
Weather History for Mitchell, SD| Weather Underground