"According to AAA, winter blend gas is cheaper than summer blend gas because it is able to contain more butane. Butane is less expensive compared to other gasoline ingredients but also helps your gas ignite in lower temperatures. However, this added butane makes the gas slightly less energy-efficient."
If this propaganda were true: then winter MPG would be lower because of the butane.
But, my cars (Explorer 4.6 V-6, JEEP 4.0L Six, Escape V-6) ALL get better indicated MPG on their CPU displays in winter, even with warmups.
I agree that my 2014 Chevy Sonic turbo gets better MPG in the winter, but it's due to the cooler temps and higher air density, plus not having the drag of the AC. Also, I don't trust the indicated MPG; I've seen 99.9 displayed before. 
I log the miles between fill-ups.
That reminds me of those first install-it-yourself gizmos for showing fuel mileage (and other things, like manifold vacuum, etc.). My brother put one in his 1972 Mustang Mach I, circa 1973. It only had a 3-digit display with fixed decimal point for MPG and when he went down some of the long, steep (Missouri) mountain roads it would creep up until it reached 99.9 MPG when he'd put it in Neutral and kill the engine, leaving the key [back] on.
More often his 351CJ (with mods) would show 16.4 MPG. He said at 100(+) MPH it dropped to 10.0 MPG, though.
The 'fuel flowmeter' was a little circle of glass tubing containing a tiny ball, with an inlet and outlet port and a photoeye that the ball interrupted as it circled around when the fuel passed through it - 10ml at a time, I think it was? That 'flowmeter' got installed into the fuel line before the carb, "low in the engine compartment where it is cooler", per the instructions. At the time (1973) it was state-of-the-art!
One of his more interesting discoveries at the time was the varying MPG from low- and high-octane gasolines of various brands. He found the best MPG was NEVER obtained with the highest octane numbers, and the Sunoco 260 pumps with the 10-octane-increment blending lever on the pumps let him get up to 18.n MPG when he used 2 'steps' above the Regular grade (which was 2 steps above their lowest grade). His engine ran cooler then, too, which led both of us on 'quests' with our Fours (his was a CB500) and Sunoco fuels to see what we could find. I kinda wish we had that sort of flexible-fuel options today.
