So why is that my gas mileage always goes down in winter?
I heard something about different fuel mix, any other reasons?
And anybody knows what is that different fuel mix means?

Winter fuel mix, cold temps, engine oil may not get fully warmed up, colder engine runs richer fuel mix. It's the same for every car.

and the fact that most folks are using their heater during the winter... I choose to only use it in spurts and allow my bun warmers to maintain my core temp.

I rarely use A/C in the summer unless it's 97+ with 75% or more humidity.

but using the climate control for heat or a/c puts more stress on the engine, robs 1-2 hp (depending on variable factors) and uses more gas when driving the same way as you do with it off or simply venting.

Heater, rear defroster, heated seats, heated mirrors...They all consume power, so fuel consumption goes up! :thumbsdow

Auto engines run optimally at around 50-60F. Cold or hotter, they're less efficient. They're especially affected by colder temperatures. Plus, driving habits differ - in cold climates, people tend to warm up the car a little longer to clear ice, heat the interior, etc. Oil takes longer to warm up, and the winter fuel mix that ignites more easily in colder temperatures also has a lower energy density. Winter tires will also have an effect, as will the fact that tire pressures, if not checked, can be a few psi lower than summertime. I agree that running the rear window defroster will burn a little more gas, but I'm not sure I agree about the heater. IF you're running the AC or the defroster in AC mode, then yes, the engine will have to burn more fuel to supply the increased energy, but if you're only running the heater (no AC), the only difference I see is that the waste heat that the engine has to reject somehow is rejected through the heating coils into the passenger compartment instead of through the radiator into the engine compartment. So, the thermostat opens a little less frequently (and perhaps the thermostatically controlled radiator fan comes on less, too). So krazyfiend, I think you have good intentions but have it backwards by using the bun warmers instead of the heater. I'll bet the heated seats take more power than the heater.

the only difference I see is that the waste heat that the engine has to reject somehow is rejected through the heating coils into the passenger compartment instead of through the radiator into the engine compartment. So, the thermostat opens a little less frequently (and perhaps the thermostatically controlled radiator fan comes on less, too). So krazyfiend, I think you have good intentions but have it backwards by using the bun warmers instead of the heater. I'll bet the heated seats take more power than the heater.

agreed, but the above would be true when the car gets up to normal operating temps...before then, you're putting some strain on the car to heat the interior as well as get the oil and coolant up to normal temps.

but yeah, my logic was flawed in my prev post.

What kind of gas mileage are we talking about here? My 05 4AT has gotten 19 and 20 mpg the last two tankfuls. It has been very cold here, and this is strictly city stop & go. With winter-blend, 10% ethanol gas (all you can get around here now). And I will admit to letting the car warm up when it is in the single digits or below zero F.

I'm curious if this is normal or not. My mileage seems really lousy, but it is worst-case.

Heater, rear defroster, heated seats, heated mirrors...They all consume power, so fuel consumption goes up! :thumbsdow

How does an electric component use more gas. Please explain that to me. Maybe that it puts more strain on the alternator, but the alternator is always running off the engine. The heater....maybe I can understand that. But, that is also an electronic item that is managed by the heater core running hot coolant through it. Any ideas???:dunno:

The more load on the alternator causes more drag on the engine so it uses more fuel to maintain the idle speed and other engine speeds. It like going up a hill vs level ground driving. As a test, when at idle, turn on the air conditioner/heater blower or headlights. You may see a slight drop in idle speed until the enrichment solenoid opens increasing the engine speed. Opening that valve means more gas. You do the same thing with your foot when driving.

Winter blend gas is the likely culprit in cold weather states.........:)

mpumas has it right about the alternator. The engine produces two kinds of energy - rotational energy to turn the crankshaft, and heat. Electricity is produced by the alternator. Producing this takes energy and is not 100% efficient.

As far as the "strain" put on the engine by running the heater when the engine is cold, any strains would be in the forms of inefficiencies caused by slightly lower operating temperatures and all that goes with it. However, using the factoid I remembered reading that the "average" auto engine generates enough waste heat to be a furnace for the "average" four bedroom home, I doubt that the heater core transfers all that much heat relative to the output of the engine.

Winter mix fuels and cold temps do us in.

My experience living in a southern state is that mpg stays roughly the same all year. I get ~26mpg in mixed city/hwy driving and >27mpg on my last tank. Any winter losses explained in some of the posts above are offset by keeping the A/C off.