I had similar mileage to Markus but winter was low 20's. Markus do you have the 09 LT or LS ? a/c and p/s with auto ? Maybe a sunroof ? I happened upon something here but I am still testing it out. There are people whom are getting some MPG and there are people who are getting none..needs to be a common connection. My 2009 has a 95 amp alternator and A/C and P/S which is being run by a tiny micro-belt. This belt is over 60 bucks up here ( and I can only buy it from one place other than GM Auto stores here are telling me they will never be stocking or able to order that belt) any other serpentine belt is 30....so it's double and darn hard to come by. There are several breaks in the serial numbers of the Aveo in 2009...small changes can sometimes make a very big impact. I am looking at the parts from the 2009 that have been carried over to the Sonic such as the coil, camshaft position sensors and seeing what's been changed. What's been changed is a great place to start looking for trouble.