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Old 01-17-2007, 12:49 PM   #1
Fast Eddy
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Default Gas mileage vs temperature

Between 100 F and around 30-40 F, gas mileage doesn't seem to be affected. But, temperatures around 10 and below drastically affect mileage. Can someone smarter than me please explain this?
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Old 01-17-2007, 12:53 PM   #2
mr. m
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Longer warm up times and gas reformulated for lower temperatures. Winter gas is formulated for easier starting and has lower energy content than warm weather formulations.
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Old 01-17-2007, 01:04 PM   #3
mkaresh
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The size of the difference should come out in my site's real-world fuel economy survey. It's more sophisticated than most such surveys, and includes a question about temperature/AC use. (Ideally these would be two separate questions, but I'm trying to make the survey as easy as possible.)

When I have enough responses, I'll be statistically analyzing the results to pull out values for all of the variables. For now, I'm just posting responses:

Real-world fuel economy survey results
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Old 01-17-2007, 01:09 PM   #4
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Quote:
Originally Posted by mr. m View Post
Longer warm up times and gas reformulated for lower temperatures. Winter gas is formulated for easier starting and has lower energy content than warm weather formulations.
That, and your car runs super rich when the engine is cold, and if it's really cold out, the warm up process takes a lot longer. If you take shortish trips to work/school/whatever, you'll notice a drastic drop because of that.
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Old 01-17-2007, 01:14 PM   #5
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Colder air means it's more dense, so your EM compensates by adding more fuel so you don't run lean.
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Old 01-17-2007, 01:20 PM   #6
RallyLegacy
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yes, but if you use less of the gas pedal because of the more dense air in the cold, then shouldn't that have no effect on fuel economy, or is there still an effect somehow?
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Old 01-17-2007, 02:14 PM   #7
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Quote:
Originally Posted by FutureRexOwner View Post
yes, but if you use less of the gas pedal because of the more dense air in the cold, then shouldn't that have no effect on fuel economy, or is there still an effect somehow?
But think about it. I know I definitely have to use more throttle when it's cold out to get the same level of performance out of the car.

Quote:
Originally Posted by MulletSlayer View Post
Colder air means it's more dense, so your EM compensates by adding more fuel so you don't run lean.
I was more referring to the engine being cold, but yes, this is true. I'm just saying that our ECU tends to generously overcompensate from the time the car starts up till it fully warms up.
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Old 01-18-2007, 02:55 PM   #8
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Quote:
Originally Posted by Wiscon_Mark View Post
But think about it. I know I definitely have to use more throttle when it's cold out to get the same level of performance out of the car.



I was more referring to the engine being cold, but yes, this is true. I'm just saying that our ECU tends to generously overcompensate from the time the car starts up till it fully warms up.
It doesn't take very long for my car to warm up fulling. I have a hard time believing poor gas mileage during warm up makes that drastic of a difference. That means I'd have to be getting about 1 mpg in the 2 miles it take to warm my car.

My mileage seems to be unaffected between the temperatures of about 40 F to 100 F. But, under say 10ish degrees F, it falls dramatically. Why? Is this my imagination or have others noticed the same thing?
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Old 01-18-2007, 03:10 PM   #9
conker69
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Have you actually collected any data concerning this??

I track all my fuel usage and the results are interesting. My MPG drops in early winter and stays that way for a fillup or two. Then it appears to return to "normal". I wonder if the introduction to winter fuel causes a response in the ECU and within 100 miles the ECU "learns" the new fuel again??
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Old 01-18-2007, 07:14 PM   #10
Fast Eddy
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I haven't written down the numbers but I mentally calculate everytime I fill up and it's dramatic. If we have a cold snap that lasts for a week and it never gets above 10 degrees F, it's very noticeable.
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Old 01-18-2007, 11:12 PM   #11
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I read an article once arguing that the dense cold air provided enough wind resistance to decrease fuel economy by something in the neighborhood of 10%. It was a pretty compelling argument. I tried searching but couldn't find it again- sorry.

I used to believe in the takes longer to warm up and is rich theory, but I noticed poorer winter fuel economy even on long trips driving straight through on a tank.
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Old 01-20-2007, 12:35 AM   #12
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I've got an excel workheet of the last few months for my mileage. When the cold has snapped here my mileage has gone down slightly. I just attribute it to not letting the car "properly" warm up before taking off. I try and let it idle for 30 seconds to a minute.

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Old 01-20-2007, 12:58 PM   #13
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You guys don't know what you're talking about. In the winter your Trunk Monkey gets cold and will run the engine at night to keep warm. This uses some fuel and is reflected in your lower overall economy.

It's a small price to pay to keep the little guy comfortable.
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Old 01-23-2007, 08:29 AM   #14
Big-E
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I vote Trunk Monkey! You do have to keep the little guy warm.

Seriously though. New cars have HGO's or Heated Gas Oxygen sensors so that 2 minutes after start-up, the ECU switches to a closed loop operation to effectively curb emissions and as a result to reduce warm-up / idle fuel consumption.

But once you start driving and the engine is not fully warm, the fuel/air mixture is affected more. Once at operating temperature, the fuel/air mixture continues to be adjusted, but the "warm-up" factor would no longer be a part of the equation.

The fact that it is winter does mean more dense air going into the engine and thus the ECU has to adjust the fuel/air mixture so that a lean condition is avoided. The oxygen sensors also adjust this fuel/air mixture for driveability and/or emissions purposes as well.

Here in the Northeast it is mandated that the gasolines sold are blended. Previously the gasolines were blended with MTBE (made to be expensive) or oxygenated fuel, for example, but are now blended with ethanol.

The blending effectively aides in speeding up the "burn-off" factor of the emitted gasses so they don't hang around at ground level during the winter. This also affects overall per tank mileage.

I think that I have effectively restated what most have said.

Oh well.
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Old 01-23-2007, 09:13 AM   #15
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Winter gasoline is blended to lower the vapor pressure. That makes the gas mix with air more readily at lower temperatures, probably has an effoct on the air to fuel ratio. Something else that has changed over the years, is fuel lines freezing. Rarely see that any more. There use to be a big market for additives to act like a fuel anti-freeze, Heet for example. The lower fuel economy may be due more to the temperature difference between ambient starting temperature versus normal operating temperature. There is no operator intervention when starting, manual choke or something, any more so it is up to the ECU to compensate.
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