You will notice decreased mpgs in colder, especially subfreezing temps. The engine has to run longer to warm up and keep the emissions equipment warm. When the engine is off at stop lights, etc, or when driving on battery for a prolonged period, everything will cool down and the engine may have to run to warm up again. Thus, you are forced into burning more gas when it is cold. Additionally, cold batteries don't work as well as warm batteries. There are other reasons for lower mpgs when it is cold, also.
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