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Poor Gas Mileage during cold weather

Several weeks ago a caller said his gas mileage dropped dramatically during cold weather. I noticed that the cars I drive get much better gas mileage during warm/hot weather. Highway driving: 33-34 summer, 29 winter. City driving: 24 summer 20 winter. I asked a professional mechanic why this would be so. He said that the mass air flow sensor senses much denser air during cold weather driving and compensates with more fuel to achieve the proper air-fuel mixture. The opposite would be true during summer driving. This would seem to make sense. Most of your answers seem to indicate thermostat problems. Does the mass air flow explanation make sense?

The MAF is also a factor, as are drivers who warm-up the engine for a longer period of time, lower tire pressure, and stiffer lubricants. Those factors account for most of the lower gas mileage that people experience in the winter.

Then, throw in a thermostat that is stuck in the open position on some cars, and you have a fairly complete picture of why gas mileage drops off–sometimes significantly–during the winter months.

There’s a number of contributing factors, but the biggest is simply that engines take a lot longer to reach full operating temperature from zero degrees than they do from eighty degrees, and cold engines run richer.

At subzero temperatures some engines will never reach full operating temp. When I was in North Dakota we used to have to put corrogated cardboard in front of the radiators to get heat. Especially on the highway. The subzero air passing through the radiator at 70 mph removed heat from the coolant faster than the engine could put it in.