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beowulf77

2021 AT4 Duramax 3.0L low mpg (~17mpg) per computer

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I have 420 miles on my new AT4 1500 Diesel 3.0L and the mpg has never been above 18mpg.... even driving home from the dealership on the highway for an hour. I've reset the trip computer and driven highway, city, whatever and it seems to stick around 17.5 mpg. I am not a heavy throttle guy at all and am very conservative on the gas and brake and all that. Is this a break-in thing? Or possibly a gas issue? I got the diesel from QT my first fillup. *EDIT* I am now reading their Cetane level is 40? I will go down this gas path a bit as it seems the most obvious.

 

Any ideas? I bought this truck specifically for mpg in the 20s.

 

Thank you

Edited by beowulf77

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My computer calculates my mpg to be lower than what I hand calculate by miles driven/gallons to top off. However, that’s usually only a small difference such as 28 computer, 30mpg hand calculated. 
So, to be sure, I would hand calculate the mpg you are getting. That said, 17.5mpg is quite abnormally low unless you are towing. The only time my mpg gets that low is towing in the city with stop and go traffic. With 1,000+ Lbs in the bed and not being light on accelerator, I can easily maintain at least 22-24mpg in the city and still hit 30+ highway. 
 

If you hand calculate your mpg as being in the 17-18s and not towing, I would definitely have it checked out. 

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Mine did the same when it was brand new.  Gas mileage began going up as it broke in.  Now I never get any less than 24 MPG on a tank of gas.

 

Give it a few thousand miles.  It will come around.

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Depending on where and how I’m driving makes a big difference. Around town and on back roads I will usually average mid 20s. Driving 80 on interstate i will see 19-20. Driving 65 into a 30+ MPH head wind then I’m down to 17. Haven’t gotten over the magic 30 number since I put larger tires on then stock but to get close cruising around town.


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1 hour ago, Sierra 3.0 said:

Mine did the same when it was brand new.  Gas mileage began going up as it broke in.  Now I never get any less than 24 MPG on a tank of gas.

 

Give it a few thousand miles.  It will come around.


thank you I was starting to wonder if it was a diesel thing or duramax 3.0 thing or what. I am new to diesels. Love it so far. I will revisit at 2k.

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10 hours ago, beowulf77 said:

.... even driving home from the dealership on the highway for an hour.

What speed do you cruise at?

The faster you go, the more wind drag. The optimal speed for fuel economy is around 50 mph. I'm guessing you were going well over 70.

The EPA fuel economy numbers are misleading. What they call "highway" should really be called "Sunday drive on country backroads". 

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2 hours ago, beowulf77 said:


thank you I was starting to wonder if it was a diesel thing or duramax 3.0 thing or what. I am new to diesels. Love it so far. I will revisit at 2k.

Tracking dozens of vehicles over decades I find mileage peaks around 35-40K miles. Be patient. 

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+1 on ...It's the computer ... 

I had a similar response ... then it all came together after a short while.

 

 

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Wow thats pretty low! Mine has been pretty consistant mpg wise since I picked it up. Have you checked it without the computer, hand calculated, to actually see if its off? Did a trip this summer first tank running 70-80mph I was right around 26mpg and next tank 55-70 all around type driving getting 29mpg.  Found my computer mpg was always lower than hand calculated by almost 2mpg at times. Also depends how throttle happy you are but found my AT4 does pretty well overall even towing. Mine has only around 5000 miles on it and hoping it does better the more it racks on more. 

 

Hope you find the cause of yours

 

-Grover67

 

thumbnail (33).jpeg

Edited by Grover67

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Posted (edited)
On 11/10/2020 at 6:30 AM, Sierra 3.0 said:

Mine did the same when it was brand new.  Gas mileage began going up as it broke in.  Now I never get any less than 24 MPG on a tank of gas.

 

Give it a few thousand miles.  It will come around.


I’m at 3000 miles and still getting 17-18 mpg. When did yours loosen up... if you remember?

 

im thinking of taking it in but seems like they are going to have trouble finding an issue with no codes. But maybe they have some sort of diagnostic they can run. 
 

also it is the same hand calculating it 

Edited by beowulf77

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Although my Silverado is 2WD, I've averaged under 25 mpg once. The computer is a bit optimistic by 1 or 2 mpg. My best is 34+ mpg. (I had to drive 50 mph in a work zone for a number of miles.)

I use an App called "Simply Auto" to record tank fills.

From brand new at 3 miles it has been 25+.

Now at 2514 miles I'm averaging 27.5 mpg by App tank full computing.

 

Wind, I've found makes a BIG difference. Heading into 10-15 mph wind speeds will bring my mileage down to 17-20. This is averaging 70 mph.

Returning and with a tail wind, the mileage is back up to 28+

A lot of my driving would be called city, around 15 miles total per trip, averaging between 35 to 45 mph with occasional 90 mile trips to the big city.

(I'm retired after all ?

 

My tank full average has only gone under 25 with 23.27 once in 2514 miles.

 

I can only hope for better after the engine is broken in.

 

I would definitely get your 3.0L checked out.

It doesn't sound right. 

But, I'm sure happy!

    

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Posted (edited)
6 hours ago, beowulf77 said:


I’m at 3000 miles and still getting 17-18 mpg. When did yours loosen up... if you remember?

 

im thinking of taking it in but seems like they are going to have trouble finding an issue with no codes. But maybe they have some sort of diagnostic they can run. 
 

also it is the same hand calculating it 

I've only got about 1800 miles on mine.

It is a sierra - they have slightly lower epa rating than the chev,  it is 4wd - they have slightly lower rating than the 2wd.

It is the season we get a winter fuel blend in this area, which brings slightly lower mpg than summer fuel.

 

It's never got numbers as low as you are talking. I don't think the shortest of the 3 average screens has ever got below 20.  Tank averages so far have been 24.5 +/- a few points.  

Most frequent running speed has been in the  55 - 60 mph range which usually gives 25 -27 mpg averages on the lowest mile DIC 'average' screen. 

 

Certainly sounds like your truck isn't performing as it should and needs some attention.  

I'd get it checked out.  

 

 

 

 

 

 

 

Edited by redwngr

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Posted (edited)

I got my truck last week and have less than 300 miles on it. The computer is showing 23 mpg.

Now I have the diesel off-road package

(3.73 gears) so I expect a little lower MPGs

Edited by monteholic

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I don't know a whole lot about this stuff, but I just read an interesting article at tfltruck[.]com: https://tfltruck.com/2021/01/owner-review-here-is-my-2020-ford-super-duty-diesel-regen-schedule-it-lasts-for-miles/

 

Could the computer be "stuck" in the regeneration cycle where it uses more fuel to raise the exhaust temp to clean out the Diesel particulate filter?

 

Might be worthwhile getting an OBDII reader and monitoring the Exhaust gas recirculation valve setting and the Exhaust gas temps just to see if anything looks out of place since you're using more fuel than typical.

 

I picked up a "BlueDriver OBDII" at Amazon.

 

That where I would start anyway.

Good Luck figuring it out.

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