I live in what you would call a salted road portion of the Northern US. I have noticed that at this time of year, and EVERY year, my MPG drops (PREDICTABLY) like a leaf falling from a tree. Literally, when the leaves fall, so does my MPG.
I notice exactly the same here in Scotland. As soon as the temperature starts to drop and made worse by wet roads. You notice that where you descend a hill on a closed throttle, you have to open the throttle earlier in the winter than you do in the summer. The tarmac is much colder, and usually the tyres are colder too. I also find that from a cold morning start I have to drive further before the temperature gets up to the normal operating level (the Jazz fas a blue light that goes out at 55 degrees C.
2006 Honda Jazz 1.2i-DSi S Vivid Blue Pearl