Winter fuel mileage?

99grandamgt

Does fuel mileage normally decrease during the
winter months? I have around 30,000 miles on my
1999 Grand Am GT and fanatically maintain it, using
only 5w-30 Mobil 1 synthetic every 3,000 miles. I
have done some intake and exhaust modifications to
improve performance, which in turn netted me an
increase in mileage to about 23 mpg in town.
However, now that winter has arrived here in
Colorado, my mileage has suddenly gone bye bye.
I'm lucky if I can get 18 mpg in town now. So I
ask again, does gas mileage ordinarally decrease
during winter? Also, they add ethynol to the gas
here in the winter to combat emisions. Does that
decrease mileage? I'm sort of frustrated, but my
car still runs awsome, and hasn't lost power or
driveability at all! I'm stumped.........

alcann

Along with various other inputs, the engine control computer receives a signal from the coolant temp sensor. It uses this to richen the air/fuel mixture with a cold engine to improve cold driveability and throttle response (does the same job as the choke on carburetted engines). The colder the weather, the longer the ECM supplies a richer mixture than Lambda (14.7:1 air/fuel ratio). Increased fuel consumption in cold weather is normal, especially for short trip driving where the engine doesn't fully warm up.

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mmcbride1

In Denver (and some other places), our gas is oxygenated in the winter to burn cleaner. This usually reduces my mileage about 10% from Nov.-April, until they switch back to regular gas.