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Old 11-16-2012, 11:48 AM   #2351
flyboy1100
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Quote:
Originally Posted by pxpaulx
Please do us all a favor and keep the crazy back and forth confined to the other thread...don't worry, we are all reading it for one reason or another.
Exactly
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Old 11-17-2012, 06:43 AM   #2352
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Latest fill up.

625.3km 100% city. 8.6l/100km actual. Avg temps 3c about to do first oil. Total mileage 6903.6km

Pretty close to the suggested numbers and that's fine with me! When it was a little warmer I was getting about 8 so not too shabby.
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Old 11-17-2012, 06:59 AM   #2353
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Tuned on 91 with a 3" cattless downpipe and intake I'm getting about 15/22
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Old 11-17-2012, 09:13 AM   #2354
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Getting 27.3 and just over 2500 on the odometer miles almost pure city driving.
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Old 11-17-2012, 12:14 PM   #2355
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I lost about 3mpg with the switch to winter gas I think, like night and day. Other than that, I haven't noticed any change in average mpg after almost 10,000 miles.
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Old 11-17-2012, 12:23 PM   #2356
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Quote:
Originally Posted by Haunty View Post
I lost about 3mpg with the switch to winter gas I think, like night and day. Other than that, I haven't noticed any change in average mpg after almost 10,000 miles.
My mileage immediately dropped 3-4 mpg on the last fillup 3 weeks ago. Temperature and weather was the same before and after. Funny thing was, it was only a 6 gallon fillup, so most of the gas in the tank was the older gas. All brand-name gas. Curious.
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Old 11-17-2012, 12:56 PM   #2357
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I just installed snow tires on my Impreza, and then immediately topped off my gas tank and zeroed my odometer so that I can see how much effect the snow tires have on MPGs.
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Old 11-17-2012, 01:43 PM   #2358
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ok i really think it has more to do with the fact that the car is running rich for longer warming up than any winter fill vs summer fill.
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Old 11-17-2012, 04:33 PM   #2359
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Quote:
Originally Posted by auskip07
ok i really think it has more to do with the fact that the car is running rich for longer warming up than any winter fill vs summer fill.
Yup I tend to agree on that. I can see that the rpm are staying higher for longer during my drive in that warm up. But overall its not a huge drop and was expected as it get colder.
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Old 11-18-2012, 01:39 AM   #2360
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Quote:
Originally Posted by Penguin44 View Post
Yup I tend to agree on that. I can see that the rpm are staying higher for longer during my drive in that warm up. But overall its not a huge drop and was expected as it get colder.
How about the first law of thermodynamics? I'm thinking about reduced mileage in cold weather AFTER the car warms up, that would have greater relative effect on longer trips. "Heat and work are forms of energy transfer." So, colder temperature would mean more energy used to maintain the engine at running temperature, leaving less for the "work" of moving the car once it got there.

Maybe there is a physicist out there who could comment on this?
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Old 11-18-2012, 06:11 AM   #2361
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^^ I don't think this is the valid explanation. I think it's simply that an engine (or furnace, or boiler, or kerosene heater, or oven, etc.) work most efficiently when they're hot. So the warm-up time is when our cars get crappy MPGs. Once they warm up, they're fairly efficient.

In the winter, it takes longer for the engine to warm up because the starting temp is lower, and because the air blowing into the engine is colder, so it takes longer for the engine to reach max efficiency. Add to that the winter blends of gas, and you take a double-whammy on your MPGs.
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Old 11-18-2012, 09:22 AM   #2362
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There are actually several factors why fuel economy suffers in the winter. The biggest, depending on your driving style, is that cold air is just more dense than warm air. That means several things are happening. For one, you are pushing more air - up to 10% more. Another reason is that as a certain volume of air comes into the engine, in the winter that has more mass so there is more fuel put in to compensate. Then, fluids are more viscous (thicker), which increases the power drag - things like transmissions and differentials that don't have cooling "systems" are more of a drag. Tires have less pressure so there is more rolling resistance.

The other reason, which may be major depending on your driving habits, is that people tend to warm their cars up more in the wintertime and take more shorter trips instead of walking or riding a bike, and it does take longer for all of the engine to warm up to optimum temperature in the winter (even though the gauge may say it's already warm).

So, the answer is: It depends...
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Old 11-18-2012, 09:56 AM   #2363
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Quote:
Originally Posted by chicagocpo View Post
How about the first law of thermodynamics? I'm thinking about reduced mileage in cold weather AFTER the car warms up, that would have greater relative effect on longer trips. "Heat and work are forms of energy transfer." So, colder temperature would mean more energy used to maintain the engine at running temperature, leaving less for the "work" of moving the car once it got there.

Maybe there is a physicist out there who could comment on this?
except for the fact that colder air produces more power because its more dense as said above. I believe that the engine may pour more fuel but not much. What ends up happening is the car leans out. (this is safe in colder weather) Same principle behind water injection. Or the timing adjusts

Last edited by auskip07; 11-18-2012 at 10:16 AM.
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Old 11-18-2012, 11:13 AM   #2364
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When the car is cold (and it is colder longer in the winter time) the engine is in open loop. While in open loop fuel is dumped into the engine at a higher rate than when in closed loop. This could add to the larger consumption during the colder months.
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Old 11-18-2012, 11:18 AM   #2365
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Quote:
Originally Posted by chicagocpo View Post
How about the first law of thermodynamics? I'm thinking about reduced mileage in cold weather AFTER the car warms up, that would have greater relative effect on longer trips. "Heat and work are forms of energy transfer." So, colder temperature would mean more energy used to maintain the engine at running temperature, leaving less for the "work" of moving the car once it got there.

Maybe there is a physicist out there who could comment on this?
When detonation occurs, some of the energy is converted to expansion, which powers the cylinders. Most of the energy is converted to heat, which is waste energy, doesn't contribute to powering the car. It doesn't matter if it's 32 degrees outside or 100 degrees, detonation will produce mostly waste heat in excess of what's needed to maintain the engine at optimal running temperature. So, no, in cold weather, the chemical reaction doesn't change to produce more heat and less expansion. The gas isn't "smart" that way.

Mostly, in cold weather, you have the issues that others have pointed out: fluids are thicker and take much longer to warm up, so have more resistance and take more power to move engine parts in contact with the fluids.

And then there's the winter gas issue:
According to this, winter gas has more butane, which has less energy, resulting in up to 1.7% lower energy output.

(Not a physicist).
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Old 11-18-2012, 11:34 AM   #2366
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As long as the engine is running at operating temperature, it doesn't make much of a difference whether the excess heat is shed through the block or radiator.

The problem I'm seeing is that the engine sometimes struggles to maintain operating temperature, which definitely has an impact on efficiency.

Most of the engine's power goes towards moving the air you drive through. In the winter, this air becomes more dense and provides more resistance.
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Old 11-18-2012, 11:49 AM   #2367
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Quote:
Originally Posted by Commander Keen View Post
The problem I'm seeing is that the engine sometimes struggles to maintain operating temperature, which definitely has an impact on efficiency.

Most of the engine's power goes towards moving the air you drive through. In the winter, this air becomes more dense and provides more resistance.
If the thermostat is working correctly the engine should stay at operating temperature without any problem.

Moving the air (wind resistance) is in fact generally the largest factor in fuel consumption rate, once you're warmed to equilibrium. If it's 50% (and that will be determined by the aerodynamics of the vehicle) and density is increased by 10%, you can expect your fuel consumption to increase by 5%, all other things being equal.

As far as colder air intake, the increased available power from the colder, more dense mixture going into the combustion chambers is offset, at steady state, by needing less weight on the accelerator pedal. It's the change in density, not temperature, that affects the available power.
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Old 11-18-2012, 12:13 PM   #2368
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Quote:
If the thermostat is working correctly the engine should stay at operating temperature without any problem.
The thermostat is working fine, but blowing cold air over the heater core can cool down a warmed up engine, or prevent an idling engine from ever fully warming up.

If you have a scan tool, see for yourself. The blue light going out only indicates 120F.

Quote:
If it's 50% (and that will be determined by the aerodynamics of the vehicle) and density is increased by 10%, you can expect your fuel consumption to increase by 5%, all other things being equal.
Aero losses increase exponentially with speed so that percentage will change with speed as well. Your 50% figure is true of most cars doing 25 MPH and rises with speed.
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Old 11-18-2012, 12:44 PM   #2369
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I have a Scangauge so will check that. I think you are probably correct in your generalization about "most cars" and 25 mph, but I'm not sure about the smaller much more aerodynamic cars like the Impreza. It might be 40 or so, due to the extra differential
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Old 11-18-2012, 05:30 PM   #2370
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I don't think it's that complicated. A 3mpg drop is quite a bit, and a 3 mpg drop right after a fill-up while the temperature remains the same between fill-ups, kinda makes me think it's winter blend gas.
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Old 11-19-2012, 01:24 AM   #2371
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Quote:
Originally Posted by Haunty View Post
I don't think it's that complicated. A 3mpg drop is quite a bit, and a 3 mpg drop right after a fill-up while the temperature remains the same between fill-ups, kinda makes me think it's winter blend gas.
That's what I've noticed in mild California during my last two tanks. There has been a noticeable drop in mileage with no other significant changes.
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Old 11-19-2012, 07:45 PM   #2372
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Cold temps... Bah...
330.3 miles. 13.04 gal.
27 mpg avg. 25.3 mpg actual.
23 mph avg.
9625 miles on ODO.
http://www.fuelly.com/driver/hemophilic/impreza
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Old 11-19-2012, 07:51 PM   #2373
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Same here. I've easily dropped about 3 MPG within the past few weeks.
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Old 11-19-2012, 09:02 PM   #2374
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I monitored the water temperature via the Scangauge as the engine heated up this morning. When the interior of the vehicle was cold (at first) the engine would in fact slowly cool at stop lights. However, when the interior warmed up the Scangauge consistently read 182 to 184 deg F.
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Old 11-19-2012, 09:48 PM   #2375
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I'm averaging 27.2 so far all my fill-up data available at:

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