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Old 12-18-2009, 01:26 PM   #1
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fuel economy question

Gas mileage is going down as the temperature goes down, what gives?

As it is getting colder I am noticing that my fuel economy is going down.
This feels strange to me since I am definitely driving smoother and pretty much taking all of the driving tips for good fuel economy, but my numbers are going down. I also check my tire pressure during each fuel up.

I have stopped using my A/C which always seems to be the #1 killer here in the desert. I was also told that diesel's like cooler weather.

What am I missing that cold be hurting my fuel economy? I'm trying to break the 41 MPG mark.
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Old 12-20-2009, 12:41 AM   #2
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Hi, im noticing the same thing. I think its got something to do with the temperature of the fuel. The warmer fuel gets, it expands, thus in effect creating more in your tank.

It will be interesting to see if the MPG starts going up towards spring and summer, in theory it should.
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Old 12-20-2009, 04:57 AM   #3
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Could be oxygenated or ethanol blend gasoline. It will be listed on a label on the fuel pump. Both can negatively impact fuel economy by about 10%.

In March these additives, especially oxygenated gasoline, will be removed and your MPG will return to normal. There's nothing you can do about that.
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Old 12-20-2009, 06:47 AM   #4
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Oxygenated blend in the winter is one factor. The other factor is that the air is colder, hence denser. That means more air enters your engine on the intake stroke. To compensate, for emission purposes, more fuel is injected.
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Old 12-20-2009, 09:03 PM   #5
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There is also winter fuel for diesels that will drop your milage and is used to keep the fuel from gelling when it gets really cold. Diesels have a harder time warming up in the morning. My TDI takes alot longer to get warm on the way to work. If you do lots of short trips then the motor never gets to operating temperature. The motor takes more effort and more fuel to get the car warmed up, from warming you up, oils and other fluids. Transmission fluids, gear oils and grease flow less in cold weather. Ask some of the guys and gals up in Alaska or Canada. They have to leave there cars running sometimes while going to the store or hey have plug in heaters for there car. Some have to use special fuel and fluids when in the deep freeze. Spring time should get you some better milage! have a good holiday.

Erick
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Old 12-21-2009, 08:52 AM   #6
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I have always noticed fuel usage increase in the Winter. Also keep in mind most people tend to let their cars idle more in the winter, warm up before they leave, etc, or leave running when they make a quick stop or *gasp* leave them running while fueling up!

I try to minimize this as much as I can, unless it is really cold (like 0F or worse) I don't let my car warm up much before taking off. But there is a downside to that too. Transmissions will not use overdrive till they are warm enough, so you may have to drive a few miles in a higher gear using more fuel until they do. So there is a balance. Those that keep them in a semi warmed garage have little to no worries about this.

In general, you are going to find your fuel usage to be higher in colder weather, regardless how much you try to conserve. I go from an average of 23MPG with the big van down to 18 during the winter and I don't really change my driving habits much, if at all.
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Old 12-21-2009, 11:38 AM   #7
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Thank you everyone for your input. It has been helpful. DTMAce, I have actually been keeping my car outside instead of my garage because I am moving and have no room. It makes perfect sense.
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Old 12-21-2009, 10:14 PM   #8
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Lots of reasons:

- Higher wind resistance

o Cold air is more dense, a 70F drop in air temp yields 10% more dense air

o Costs more power to move the same speed on the hiway.

- Higher rolling resistance in the cold

o More viscous lubricants, transmission, bearings, engine oil

o lasts until lubs get up to temperature. (wheel bearing and diff never does, tranny takes a long time)

o Tires deflate some in the cold, check tire pressure when temp goes down.

o Tires get harder in the cold, more rolling resistance.

o If you change to winter tires, they also have more rolling resistance then summer tires.

- Engine efficiency goes down.

o Fuel doesn't vaporize as well in a cold engine. Remember, fuel is exposed to outside air temp always.

o Denser air, due to cold, actually can yield a bit more power but that generally yields less efficiency. (not really sure how much effect this has)
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Old 12-22-2009, 12:20 AM   #9
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Great info Dilkie and Dtmace! This is why I keep reading Fuelly!
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Old 12-27-2009, 09:10 AM   #10
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On my commute (45 Miles, one way), I've noticed its not the speed I go, but other things that cause worse FE.

Like, having a head (or cross) wend, traffic (if everyone is passing me, the drag goes up and down (I go at least 5 MPH over), A/C on or off (AC is on for the Defrost and maybe the heater), and more in-town driving...
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