The setup: 2006 HCH-II, ~50,000 km on it (35,000 miles), perfect service, winter tires. I drive 65km to work (50 miles), 40-45 of them hiway.

Monday morning, sunny and warm, around 3 degrees (38 f). Drive to work and easily get ~5.2 l/100km (45 mpg). Storm comes in during the day, temperature falls to 3 below (28f). On the drive home I get 6.1 l/100km (39 mpg).

So a change of about 6 degrees Celsius results in a 20% drop in fuel economy? Why? The car is fully warmed up 5 minutes into the 50 minute drive. What's going on?