[MetroMPG]

"Winter" and "summer" is all relative.

You're talking about a 40% increase in fuel consumption going 10-12C cooler, and still remaining well above freezing. I can't see temperature being the only culprit in a change that big.

Unless you also get a big change in prevailing winds/speed with the seasonal change, but you're talking mostly about short trips (low speed, I assume).