does cold weather affect gas mileage?
 

The gas mileage of my chevy spark has gone down since its been cold.

Yes. Engine warmup periods are longer, cold air is denser requiring more fuel, and winter blend fuels all combine to result in lower economy in the winter.

Plus, tire pressures drop. So check and make sure they aren't low. Use a gauge, eyeballing doesn't work. By the time a modern tire looks low it is nearly flat. The TPMS will pick a low tire up before it looks low, but they have a wide enough range to not report a tire that is slightly low.

Economy in my Saturn went down around 5 mpg from summer to winter.

Cold engines run RICH until they get up to normal operating temp.

Use the front defroster only when needed. The AC compressor will run even though the AC button is not on.

There's also more time spent in traffic jams on slippery freeways, increased rolling resistance on fresh snow, and lots of increased electrical usage for headlights/fan/etc.

That's only partly true. The AC low pressure switch will keep the compressor from running when the temperatures get down near freezing. The temperatures vary with the unit, but usually about 40F or 5C will be the cutoff. The idea is to keep the evaporator from freezing up.

Of course, if the compressor DOES run it provides a bit more load on the engine, which will then warm up more quickly.

I agree, but I know a lot of people that leave on defrost ALL the time & don't realize how wasteful it is.

turning the heater fan off till to car gets warmed up seems to help.
but it makes the car cold inside and the windows fog up sometimes

Leaving the heater blower off until the engine warms does help the engine warm more quickly. In truly cold weather there is a hazard involved in leaving that motor off. If the defroster is turned on after the engine warms, the blast of hot air hitting the cold windshield has been known to crack the glass. Decades ago Americans stationed in Alaska were told to turn the defroster on as soon as they started the car, to avoid that very problem.