Q. Why do engines get lower fuel mileage in winter? I thought that colder,
denser air would allow for better economy.
A. There are many reasons. In
cold weather, the engine is in warm-up mode longer, which means the fuel mixture is richer.
Cold air is denser, but that requires more fuel to be mixed with it to maintain the proper
air-fuel ratio. Winter-blend gas contains more butane (which has less energy content) to
raise the Reid vapor pressure for better cold starting. The lubricants are thicker when
cold, requiring more power. Also, using the defroster (which runs the air-conditioning
compressor) uses power.