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Old 02-20-2016, 09:45 PM
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should mpg go up when its cold?

Should my mpg go up or down when it gets below 40* ? Mine goes down.

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Old 02-20-2016, 09:46 PM   #2
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The air is more dense, you are taking in more oxygen. With that, you are burning more fuel. If you are in a region that switches to a winter fuel blend, it's even worse.

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Old 02-20-2016, 09:59 PM   #3
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Adamo99 is correct..
if you go up in altitude you will get better mpg. the air is thinner and thus less oxygen so your computer will reduce the amount of fuel needed to combust.
that is about the only time you will see your economy increase. of course that is on level ground. if you are climbing a hill it will reduce because you need to push the pedal harder to maintain speed..

short version is that the more oxygen read by the computer the more fuel it thinks you need to burn for combustion..
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Old 02-21-2016, 08:54 AM   #4
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Adamo 99 is correct also. Idle time warming up the vehicle. Blasting the heat all decrease mpg
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Old 02-21-2016, 09:32 AM   #5
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Down...down... especially here in the Northeast!
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Old 02-21-2016, 09:59 AM   #6
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I think main factors for winter mpg decrease are:
-winter fuel blend
-running defrost (AC)
-Tire pressure decrease due to temps (make sure to check tire pressure)
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Old 02-21-2016, 10:11 AM   #7
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I think the freezing brain chiggers that rain down in the winter decrease the frictional coefficient of the roadway and with my hyper-powered Jeep I fry the tyres even more than during the summer. This leads to even more RPMs being used up for spinning rather than forward motion causing me to ignite the stock 2-stage gasoline powered rockets to help produce forward motion further exacerbating the mileage issue. I don't think I would have this problem if I had something as sedate as a wimpy Hellcat engine in my Jeep, heck I think FCA should look at that as an option - they can claim an increase in their CAFE too. Win - Win 8)


If the ECU sees less available air it will open the throttle body more and add the appropriate amount of fuel for it as required by the throttle position.

Usually in the winter people idle longer to warm up the vehicle so they have warm air coming out of the vents and this burns fuel. Also, for people that don't use the A/C much in the summer the defroster runs the compressor, possibly affecting mileage. Or it may have to do with the lower temp of the fuel.

Solution..... Hellcat
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Old 02-21-2016, 10:39 AM   #8
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thermodynamics vs Big Brother (Big Brother wins!)

A gasoline engine is a "heat engine." The larger the difference in temperature between the engine and the ambient, the larger the potential thermal efficiency SHOULD be for the heat engine. So, in theory, you should be running a more efficient engine in the winter than summer (on average) if nothing else has changed and as a result you SHOULD get better mpg on average, but not much.

But, all things are not equal in summer/winter thanks to Big Brother. There is the EPA mandated "winter blend" of gasoline in the USA. Has to do with adjusting the Reid Vapor Pressure (RVP) of the fuel so its a more volatile gasoline (higher RVP), and easier to evaporate in the winter for starting, etc., and for emission control/reduction at start up and during warm-up. As a result of this RVP and other things, winter blends provide less energy/volume. Less energy/volume = lower mpg.

The EPA has stated on record : "...summer-blend gasoline contains 1.7 percent more energy than winter-blend gas, which is one reason why gas mileage is slightly better in the summer."

So, there you have it - the Gov says so!

Begs the question whether the RPV of SoCal or Miami gas needs to change as much as Boston, MA or Green Bay, WI in the winter (why should it?) or is this just a ploy to get more ethanol in the gas everywhere..... ? That's another thread for another day
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Old 02-21-2016, 08:37 PM   #9
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My brain is twisted on this... If there is more oxygen, and therefore more fire burning, wouldn't that create more energy with less gas? If the engine becomes more efficient at creating energy, doesn't that decrease the amount you need to press the pedal down? (even if it's an unnoticeable amount?)
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Old 02-21-2016, 08:38 PM   #10
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Oh, and hi. I had forgotten I had never posted on this account. lol
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Old 02-21-2016, 09:26 PM   #11
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if you remember to change the blinker fluid.. (be sure to use the winter blend so it doesnt freeze)
and you remove the summer air from the tires and put the winter air in you will be fine.. MPG should improve by at least 10 MPG...
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Old 02-21-2016, 11:21 PM   #12
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Originally Posted by JKtidefan View Post
My brain is twisted on this... If there is more oxygen, and therefore more fire burning, wouldn't that create more energy with less gas? If the engine becomes more efficient at creating energy, doesn't that decrease the amount you need to press the pedal down? (even if it's an unnoticeable amount?)
If more oxygen than is needed is provided, it doesn't make more energy - it only gets all the available fuel energy released. A given Fuel has a fixed amount of energy/gal. Winter fuel has less energy/gallon, so less energy is/can be produced per gallon. The ECU will adjust for this by controlling the fuel/air ratio to try to get the SAME energy for each spark. It injects a fraction more winter blend fuel to be burned to get the same amount of energy so that the engine can produce the target camshaft rotations per minute (horsepower). Hence, using more fuel to get the same energy results in lower mpg. Same thing sorta happens with going from 87 to E85.

The difference in energy content btween winter & summer blends trumps any "efficiency gain" of our heat engine when its used in cold ambient.


ktwillys - lol. Have you tried using 45psi of Nitrous in 37" tires in the winter, and getting it nice and hot, and piping it into the optional "penta-barrel carb" of the Pentastar V6? Awesome! best mod ever!
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Old 02-21-2016, 11:25 PM   #13
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KickASS! Great response, man. I definitely forgot to take into account the computer. The rest of that really helped.
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Old 02-22-2016, 10:03 AM   #14
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Another reason for cold weather fuel economy reduction are all of the fluids become "thicker" in the cold. They do not flow as well and require more energy. This is one of the reasons synthetic fluids are better in cold climates.
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Old 02-22-2016, 11:08 AM   #15
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All the discussion about colder air being denser and the computer injecting more fuel to match as an explanation for lower MPG in cold weather is incorrect. That is a good explanation for why an engine can produce more power in colder weather, but when driving on the street, you don't use a fixed throttle position and accept whatever amount of power is produced at that throttle position. While driving, you are constantly adjusting the throttle position to give you whatever amount of power you need.

Ignoring everything else for simplicity, and only thinking about the colder/denser air's effect on the engine, you would need less throttle to accelerate at a given rate, or maintain speed, when the air is colder. At smaller throttle openings, there is increased vacuum pumping losses. More of the engine's total power is wasted to simply suck air past the smaller throttle body opening. So this is one factor explaining lower MPG in colder weather.

Related fun fact: Some vehicles have heated air intakes, where hot engine coolant is routed through the air box to heat up the air. This is done for improved fuel economy, because it requires the throttle to be opened more to produce sufficient power for daily driving, which reduces vacuum pumping losses and increases overall efficiency of the engine (under daily driving conditions). On these vehicles, a simple modification to re-route the coolant around the air box gives a quick/cheap gain in power.

At higher speeds where air resistance becomes substantial, also consider that colder/denser air produces more drag.

Then there's other factors that have been mentioned already, like longer warm-up times (engine runs intentionally rich while the engine is cold for various reasons, some people leave their car idling for long periods of time in cold weather for comfort reasons, etc), cold lubricants being thicker (engine oil, transmission fluid, gear oil in differentials), and winter blend fuels containing less energy density.

If most of your driving is relatively short trips to/from work every day, where the engine completely cools down between each trip, then MPG will suffer more in the cold than longer trips, or more closely spaced trips where the engine is still warmed up when you start up again. The larger the portion of your time/miles spent idling/driving with a cold engine, the bigger the impact of cold weather on your MPG.
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Old 02-22-2016, 11:26 AM   #16
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No MPG will not go up when cold. There are a multitude of reasons for this. They include:

1.) Engine requiring extended warmup periods. This is the largest contributor to lost fuel economy. If you want to prove this to yourself get a scangauge (or equivalent) and calibrate its fuel economy readout to your vehicle. Test your fuel economy over a standard route (multiple times) in the summer, in the winter starting cold, and in the winter once everything is warmed up. You will see the difference from summer to winter is marginal once the engine is warmed up.

2.) Along with point 1 all other fluids, bearing grease, diff fluid, etc. take longer to warm up and reach their target viscosity levels.

3.) As mentioned above the air is denser, this results in marginally more air resistance (will only have an effect at highway speeds).

4.) As mentioned above winter blend fuel. This has marginally fewer BTUs in it than a summer blend. The difference here is quite real but much less than some would have you believe.

5.) If you have snow on the ground there is a reduction in friction between the tire and road, resulting in an increase in slippage and wasted energy.

6.) There are probably a few I've forgotten or left out. Either way the main culprit in lost winter fuel efficiency is the engine needing to warm up, I would encourage everyone to test this themselves if they don't believe it.
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Old 02-22-2016, 12:28 PM   #17
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No, more oxygen would mean your engine is running lean. To counter that the computer routes more fuel to balance it out. This means you burn more fuel for the same energy. At least that is how I understand it. I asked the dealership up here the same thing because, in Alaska, my winter fuel mileage for a stock JK is between 12.8-14 mpg in town.

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Old 02-22-2016, 09:05 PM   #18
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X ounces of fuel compressed with the correct mass of air gives So much energy. It doesn't matter where your foot is on the throttle, it doesn't matter if you are at 200' or 6000'. As long as you put X ounces of fuel and the correct mass of air there is the same amount of energy available from the reaction. As long as you don't change the operating parameters of the engine at the same RPM you will produce the same power - unless you put a different fuel into the equation. A lot of things can (and do) affect MPG. If you don't adjust your cold pressure in your tires mileage will suffer because the tires are less efficient at lower pressures leading to heating in the tire and air. This heat is from internal friction and takes horsepower to generate. Your heater takes some of the heat from combustion and converts your cab into a more comfortable space, this also adds a bit of load to the water pump to move the hot coolant into the cab and these consume horsepower. The fan for your heater does too. When warming up the injectors pump extra fuel in the cylinders to allow the engine to run as not all the fuel atomizes which would lead to the engine not running or running lean (just because you don't pull a choke lever doesn't mean the equivalent isn't happening. When you start the Jeep and run back inside while it warms up - more fuel used but no miles moved so MPG goes down. And as stated above some places use a winter blend of fuel with less energy content (see earlier) requiring more fuel for the same performance (including idle) than summer.

I still think FCA should give us another 450 - 500 hp (over my 3.8) to solve the problem... Or at least it would make me forget about it
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Old 02-23-2016, 10:00 AM   #19
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X ounces of fuel compressed with the correct mass of air gives So much energy. It doesn't matter where your foot is on the throttle
Yes, and no, depending on whether you are talking about total amount of energy released, or net energy output of the engine. Yes, the total (gross) energy is the same, but net output of the engine is different when a smaller throttle opening is necessary when the air is colder. Energy is used to pull air through the throttle. Smaller throttle opening requires more energy. This is commonly referred to as "pumping loss", and more specifically, the component of pumping loss caused by part throttle operation.
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Old 02-23-2016, 11:23 AM   #20
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The air is more dense, you are taking in more oxygen. With that, you are burning more fuel. If you are in a region that switches to a winter fuel blend, it's even worse.
Live in "Splendid" Ca, with its to die for fuel blends. In the winter with stock tires I get 14.8 mpg! I had this argument with my older twin brother(9 min) he was adamant about it going up. I had to show him multiple times. He has an ego with it's own social security number.

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