Don't forget the mud would freeze so bad over night it would be rock solid around the road wheels and tracks and in the morning the tanks couldn't move.
And it has to get REALLY REALLY damn cold for gasoline to freeze! Gasoline will still ignite at -97 degrees faranheit however for it to freeze solid it would have to hit -200 to -300 degrees depending on the mixture which is almost absolute zero kelvin.
However the oil would gel up to the point where it wouldn't be able to be pumped by the oil pump in the engine. Not to mention the Germans always prided themselves on tight tolerances in their engines, throw one of those tight by design engines in temps as cold as a Russian winters night and guess what, now that tight engine is a solid chunk of metal!
Don't forget the batteries also! What happens to them when it gets damn cold out? They can't put out the amount of power they were supposed to and half the time would be dead due to the cold.
Basicly the Germans just were not prepared to deal with that kind of cold.