Originally Posted by Humble Leader

I don't think there's a problem here. the short trips at low speeds and the engine running rich and I believe 'open loop' before the O2 sensors warm up is what I think is the story here. plus colder weather means more dense air and more wind resistance. engines take longer to reach normal operating temperature and there is more drag. all the little things can add up to a fair amount of mpg reduction

for me, I get worse mileage in the winter. I also idle a lot in the winter when shoveling snow to let all the snow/ice melt off the windows. you probably don't have this problem though in san Francisco. right now I'm getting about 15mpg mostly city and some highway in the middle of winter.