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Old 07-19-2008, 08:03 AM   #1
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Gas mileage and the temperature?

Alright, I'm not an expert whatsoever on cars, so that should explain why this sounds stupid if it is. First of all, cars get better gas mileage in the summer than in the winter, right? I started thinking about this as my current tank of gas is seeming to be the best tank I've had since hypermiling. I wondered why it is so much better than the last few, especially since it includes running late for work one day and throwing the hypermiling out the window (driving 70 in my detour of a long and slow train) for a few miles. So I wondered if this being the hottest week of the summer had anything to do with it (around 90 degrees for several days now). So, does it?

Also, what about the temperature exactly causes the better FE? Does it do something to the gasoline or the engine or what? Or is what I'm saying not even true to begin with (that FE is better when it's hotter). If temp does improve it, is it not possible to USE this fact? Obviously HEATing the GAS tank sounds like a pretty bad idea, but you'd think with modern technology, the car companies might be able to come up with some sort of safe way to do this. Or does it have nothing to do with the gasoline and maybe it just deals with the engine when starting up.

Keep in mind I know next to nothing about cars so forgive me if this is all sounds retarded.

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Old 07-19-2008, 08:55 AM   #2
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Engines get up to operating temperatures faster in the warmer months, which means less gas wasted on a cold engine. Air density also decreases nearly 14% between 30*F and 100*F, so aerodynamic drag (and the horsepower required to overcome it) decreases accordingly.

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Old 07-19-2008, 09:28 AM   #3
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Woow what?

Aerodynamic drag doesn't increase it decreases in the summer. Think about it, the air is far less dense because its warmer.

Oh, and have you heard of warm air intakes? That basically makes your winter driving somewhat more like summer driving (and we all know WAI's give you better mpg)
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Old 07-19-2008, 10:38 AM   #4
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All engines should get reduced mileage in the winter from cold starts and friction but on some engines the reduction is too small to measure. From the few I've seen those that keep their winter mileage all have WAI built into the engine design. Get a Scangauge so you can watch how IAT affects mileage.

Unmodified fuel gets good MPG when the IAT exceeds 90*F, and at the same time you are turning on your AC which hides the improvement. Modified fuel may extend that temperature range a bit.

There's no consensus for why heat improves mileage. IMHO (in my hick opinion) the reason is that heat improves vaporization which makes the fuel burn faster. Faster burning fuel burns earlier in the stroke with more pressure and less heat which results in more of the work you want and less of the work you don't. How it works doesn't matter. Push up that mileage any way that works.

Carburetor cars used heat to prevent icing and improve cold weather emissions and driveability. Acura and Volvo have regulated WAI on some FI models though as it always is, it is probably for emissions and not for mileage. Sometimes they fail hot and really reduce the mileage.

I didn't find any benefit from fuel heat though some do.

As for why they don't implement WAI? The easier it gets to obtain beyond EPA MPG with minimal changes and the more I observe how better MPG affects the engine the more obvious this and similar claims become.

Originally Posted by Louis LaPoint at
The biggest probable reason for the grief and fear caused by good mileage to the oil companies and auto companies is overproduction. There is no shortage of car production lines or refineries. They simply fear that the robotic production lines are not being utilized enough, according to the Wall Street Journal. People who own cars with good mileage tend to keep them. Cars with good mileage stay on the road until they fall apart.
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Old 07-19-2008, 12:21 PM   #5
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Originally Posted by trautotuning View Post
Woow what?
Just a typo, but it's all fixed.
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Old 07-19-2008, 12:41 PM   #6
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As others have said, there are a variety of reasons why FE drops in the cold (the additional time for the engine to "warm up" being one key one). However, in addition to the things others have mentioned, here are some other reasons I think are related to lower FE in winter:

1) In winter, you tend to get "winter formula" gas at the pumps (i.e. the gasoline at the pump is not the same mix of fuels that you would be able to buy in the summer). And for some reason, the mix of gasoline that is considered "winter formula" tends to result in slightly less fuel efficiency. Yes, "winter formula" gas is supposed to make your cars slightly more resistant to stalling out in the cold, but that comes at the price of the gas itself not burning (in a number of engines) quite as well. Still, you are stuck with this in most cases, as the gas stations generally won't even give you the option to buy "summer gas" in the winter (but will instead just switch the gas formula on you, without even mentioning that they did so).

2) In winter you tend to run the accessories (including the power piggish headlights) more. And has been pointed out before (on this forum), the more electricity you use the lower your fuel economy (due to the extra alternator drag on the engine to make the extra electricity). And while in many ways you have to do this (the headlights are needed after dark, for example, and it gets darker sooner in the winter), even in winter there are still ways to conserve electrical power (such as converting many of your car lights over to energy efficient LED modules). Still, while this isn't a temp thing per se, it is one reason why FE does drop in the winter.

3) And related to the accessories is things such as the cabin heater and defroster, both of which use electricity (which will lower FE some) to run. And in the case of the cabin heater, you sometimes get a double-whammy, as it pulls "waste heat" from the engine (to heat the inside of the car) and if/when the engine isn't fully warmed up first (which all too frequently happens in the cold winter days) that can actually prolong the FE hit from the engine being "too cold" (i.e. the engine being colder than would be ideal for FE).
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Old 07-19-2008, 12:43 PM   #7
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This is a cut n' paste from a post I did on a site where we were talking about air drag ratio changes due to temperature changes. /////////////////

I'm not much when it comes to calculating air/wind drag or drag thru fluid temp swings. However, I did a lot of fuel tuning over the years that had to be very exact for snowmobile & automotive drag racing.

Generally, we found for every 10 degress F, up or down, about a 1.5% variance in fuel flow was required to maintain a similar burn. IOW what worked at +50 F had to be increased about 7.5% at 0F.

Humidity also played into our adjustments but that factor is less, very much less.

Likewise altitude has dramatic changes. For every 500 ft increase in elevation, fuel had to be cut about 1.5%.

These figures were usually pretty close and were to be taken with great care as we tuned right on the edge. Plug color and pyro readings were used in all cases as we leaned to max power.

Racing aside,

Carbed engines tuned for general specs will usually show a better mileage rate with lower temps as they're running towards the lean side as temps drop. An EFI system will usually show lower mileage as they add fuel to compensate for temp drops.

So in summary,

If you're working in 100 degree weather and you have a known torque/HP & fuel flow rate. At 50 F you'd normally need at 7-8% more fuel to maintain torque & HP, generally speaking.

In the real world, away from fine tuned racing, this tends to work fairly close. Most of my vehicles will drop 5% in fuel economy from summer, 70-90 F to winter, Zero- +30 F, when running similar fuels.

Add ethanol? Basically a 10% ethanol mix lowers fuel efficiency 3% from what regular gas offers.
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Old 07-19-2008, 04:57 PM   #8
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Look at the equation for aerodynamic drag. It has a "rho" factor that is the density of air. Hot, humid air is a lot less dense than cold, dry air. In fact, below freezing the ability of air to hold water is so low as to make "relative humidity" a joke. Pros think absolute humidity at this stage.

Also inclement conditions inhibit fuel-thrifty driving. Just getting there without bent-up sheetmetal becomes first priority.

Summer of the Gulf Coast (hot, humid, flat) should give the best MPG if you can live without A/C.
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Old 07-20-2008, 04:14 AM   #9
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I think fuel temp has alot to do with it. I do lean burn tuning. On really cold mornings I get lean misfire untill the engine warms up, even at richer mixtures.

But if the car has been in the sun but still cold temp it will have no such misfires at leaner mixtures.

I can only deduct its the fuel. So to test this theory gotta get some sort of fuel heater done for those cold mornings.

I might even chuck some acetone in to see if it helps things on those cold mornings.

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