My Ram Computer,like many others is a notorious lier. With over 100K miles, and every refill hand calculated vs the "Lie-o-meter, We are rarely within 2 mpg difference. The computer "always" reads higher.
Our worst when towing was a week ago running west on I-80 across Nebraska into a hard headwind... 7.4mpg. The best was around 12 mpg....obviously,a tailwind! Our overall towing average is just below 10mpg (hand calculated). Our fuel mileage increased "slightly" after we "tweaked" exhaust and computer. Not too shabby when "dragging" around 20K lbs. memtb