I've noticed recently that virtually every car review I read mentions how the fuel economy is so much worse in winter, often never coming close to the Energuide ratings. One of the worst was today's Honda Accord review, they only managed 15l /100km - that's worse than many SUV's!
I've owned my car for 4 years and always track the fuel consumption each time I refill the tank. I'm in Ottawa and I get roughly 8.5 l/100km all year round with mostly highway driving. If I use a full tank on the highway, I get 7.8 - 8.0 l/100km which is exactly (7.8 ) what it's rated for by Energuide, and that's at ~115km/h. However, there's no correlation with the time of year and the actual consumption. By the way, I use winter tires (in winter).
My car's a 2001 - have emission controls changed so much recently, e.g. to heat up the cat faster, that I can expect my next car to have worse economy in the winter?
This isn't an "economy car" by the way, although it's 185HP is pretty low by today's standards

Cheers,
Paul