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?

 

Air is typically 50* colder in the winter, on average. Colder air is denser, to get a correct air/fuel ratio you ECU compensates with more gas.

An example would be to take a ballon from inside your house @ 72* and bring it outside at 22*, the ballon will shrink a huge amount.

 

Besides the ECU adjusting for denser air, the main reason is a two part answer.
Number one, your car takes longer to reach optimal operating temperature.
Number two, your car is in closed loop mode, i.e. "high idle, rich mixture" for the duration of the warmup period.

To break it down:

Higher idle for a longer period of time.
Richer mixture during high idle.
Rich mixture after warmup to adjust to denser cold air.

Also a factor, but not as significant, has to do with the aerodynaimcs of pushing a car through colder denser air. It take more HP to push a car through 20 degree air than it does through 80 degree air.

 

Yeah, forgot about the warmup factor. If you let you car warm up for 15 - 20 min before you drive it, that's 15-20 minutes at 1.5k RPM. If you can stand a little cold, if you just start your car and drive it your car will warm up hella faster and you will be using the gas to get you somwhere. Most people don't understand that 15-20 min. @ 1.5k RPM+ driving to work @ 3k RPM is a lot more gasthan just driving to work @ 3k RPM + a little cold.

 

Many parts of the country have 'winter blend' gasoline (required by EPA based on pollution measurements). It has oxygenators (often alcohol) for exhaust emmissions during cold-starts. But that blend usually contains less energy per gallon.

Just as a technicality, once the engine's warmed up, the mixture isn't 'rich'. More correctly, it's the same air:fuel ratio but theair is more dense. Need more gas to prevent the mixture from going lean.

But the cold inlet air actually makes your engine a bit more efficient.

And then there'sa pretty big collection of other small things. Most of them cause higher gas usage.
Dense air = more aero drag.
Cold= more drag in wheelbearings & suspension components.
Snow on the road = more physical drag.

Do you allow your tire pressures to get low because of the cold?

 

I'm runnin 41 PSI on the Kumho's, 38 PSI on my Toyo's, and 32 PSI on my Blizzak's... All 4 PSI under maximum.

 

I'm only generalizing about the tire pressures. Sounds like you actually pay attention. There's lots of people out there whocheck their tires once every 6 months (whether they need it ornot...)

 

Ummm...... yeah.......that would be me. I always forget to check my tires. Unless one looks flat.

 

i check regular enough... now my wife.. that's another story!

i'm half tempted to get those stem-cap replacements that show green/red depending on the inflation