What a difference a day makes.
Drove from Chicago to Bozeman over the weekend, I stopped in Fargo for the night. Here's the contrast in Saturday and Sunday.
Saturday, stopped on my way in Hastings, MN to meet a friend, had time to kill since he wouldn't be done working till 6pm, so I took my time.
Temperature 60-65°F, 10-20 MPH tail wind, Avg speed, 55 MPH, took state highways for a fair portion of the journey instead of freeway so some hyper mileage techniques could be implemented.
Fill up MPG.....42.4 (513 miles on the Tank, a record best)
Sunday, it snowed during the night, I had 750 miles to go to be home, I did not care about fuel mileage that day. Screw Hypermiling...it's go time!
Temperature 12-20°F, 20-30 MPH headwind, Avg speed 75 MPH, Freeway cruise control set on 78 MPH.
Fill up MPG.... 27.2
Just another reason why anecdotal evidence of fuel savings is crap, same car, 2 days apart driven in 2 different manners and 2 different environments.
So you could view this as a 56% increase in fuel mileage or a 36% decrease, all due to how & when the car was operated.