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My 2018 Eco Diesel engine(engine replaced at 41K miles) has a noticeable drop in MPG when temp goes below 21. Just returned from two trips where I drove through areas where the temp ranged from 27 to 34 then down to 15 and then back up to 35. I averaged 29 MPG the first 92 miles. (manual calculation) when the temp dropped below 21 the MPG dropped to an average of 18 MPG. MPG went back up after temp increased again. My 2500 5.9 always ran better in colder temps. Is the ECO diesel different or is it just my truck?
 

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Curious what the duration of the 21 degree driving was...maybe a regen? Refill the tank? Otherwise, yes that drop seems excessive unless you were driving into headwind. I notice a drop in cold temps but not nearly that significant.
 

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Lots of questions. Personally I think you're making more suggestions than you probably have information to.

How did you manually calculate the 29 mpg for the first 92 miles? Are you using winter blend fuel? Like the poster above how long did it drop down for? Very possible it could have done a regen. Are you using a front cover? In those temps I believe it is recommended to. If you don't the truck wont hold operating temp and will surely drop economy.
 

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When its cold most of us warm up our trucks, and that is the number one killer of my mileage. The colder it is, the longer it takes to heat up.

At -22 F the other day, it really takes all of 15 minutes to get the inside warm enough to even hold on to the steering wheel. And the short distances we drive really throws off my mpg, it gets as low as 16-17 mpg using the EVIC.

Winter fuel is my number two reason for mpg loss. It makes a difference going from #1 blend to #2.

I use the block heater, fast idle option, Diesel Kleen for Winter fuel additive and have the front grill cover. And it still takes quite an effort to get my engine fully hot when it's below zero, thus my number 3 reason is not having my engine fully hot, at those low temps it can take an hour or more, driving on the freeway is the fastest way to get hot, a must when regenerating, another variable. If stock, you won't know when it is happening. With the regen notice option, using the EVIC, it is very noticable the drop in mpg, reason number 4.

So the answer is complicated, but in short, in winter, expect a mpg loss.

Being on my second winter with my Ram, I now know the mileage will return with the sun, so not to worry.
 

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Well I don't know why someone hasn't chimed in to say something about the winter blend of diesel fuel. It is somewhat "diluted" to help prevent it from getting waxy and not flowing well. I'll call it diesel #1 for the winter blend and diesel#2 for the rest of the year.
 

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My 2018 Eco Diesel engine(engine replaced at 41K miles) has a noticeable drop in MPG when temp goes below 21
Do you have a winter front on? and how much of it are you blocking off when it gets into the 20s?
These trucks like to be warm to get good fuel efficiency. I usually have just the lower two openings restricted when it gets close/into the twenties
 

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Well I'm starting my 5th winter with mine,
there is a noticeable decrease in mileage with the colder temps.
I have seen any wheres from a 2 to 4 mpg drop from temperature changes.
That can occur within the same tank of fuel.
This happens every winter.
Also years ago the fuel vendors would blend #1 (kerosene) with # 2 fuel oil for a "winter blend" fuel,
they have pretty much eliminated that practice and just add additives to #2 anymore, it has too be
forecast to get below zero before any of them will blend #1 and #2 now days.
So unless your fuel vendor is anticipating extreme cold your fuel will not have any #1 blended in.

When I fuel my tractors with fuel in the winter it is blended at my tank the delivery driver will put in
#1 fuel in the tank then switch compartments and finish up with #2 to what ever blend I have requested.
I also treat each tank of fuel with anti gel additives.
This has been the procedure for many years, in excess of 50 for me so I would expect its been since the 40s
or 50s when diesels started to be common.
There is a definite decrease in power and an increase in fuel consumption in the tractors also,
much of that can be attributed to the blended fuel.
If it wasn't such a royal pain and potentially hazardous situation and so costly I would not blend my
tank of winter fuel i would just use additives.
I have even contemplated getting a barrel or 2 of #1 and blending when I fueled up when if needed.
 

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Well I'm starting my 5th winter with mine,
there is a noticeable decrease in mileage with the colder temps.
I have seen any wheres from a 2 to 4 mpg drop from temperature changes.
That can occur within the same tank of fuel.
This happens every winter.
I witness the same phenomena every year, too
 

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Clear #1 diesel gives lousy mileage while towing on my truck, about 10.5 mpg.

Same load, burning that rich stinky Louisiana #2, I got 16.4 mpg.

Yea, fuel matters.
 
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