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Loafer987

Break-in time vs bad fuel economy on brand new truck

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Hello all! I've been reading posts on this site for the past 2 months or so to learn as much as possible about my 2018 sierra 1500.  I'm having a horrible time getting fuel consumption under control and I'm extremely confident my "old man" driving should be netting me much better fuel economy than what I am getting. 

I have a 2018 gmc 1500 sle elevation edition w kodiak package. crew cab, reg box, 5.3l 4x4 6 speed.  At the dealer I added the leveling kit, oem running boards, removed the air dam and upsized the tires to 285/60r20 on the stock wheels.  I added a hard tri-fold tonneau cover recently as well. I've never owned a new vehicle before so all of this is a learning curve for me.  The dealer keeps reassuring me my mileage will come down in time after break-in etc but every tank is a tiny bit worse and it's getting a bit concerning. 

According to my cluster and GMC app I have averaged 16.1 l/100 km (14.6 mpg) since the truck was new.  Now according to my Drivvo app I am averaging around 18.0l/100km (13 mpg) which is also what my math comes out to when I calculate it myself after the past few fillups.  I realize I have a few "bad for economy" mods on the truck but really my questions are:

 

1. Having only 3500kms on the truck so far, is the lack of break-in really the problem?

2. Has anyone else noticed the fuel consumption display in the truck being off by this much? 

3. Should I look at a tuner/tune to update the computer with the proper tire size and try for a tune that's better on fuel? 

 

I dont plan to tow anything with this truck other than dirt bikes in the box in the summer time, and I realize it's a truck not a car etc etc, but I have read enough posts to know I'm using almost 40-50% more fuel than the average person on here so I really just want to know what could be the issue considering the truck is literally brand new...

I am realistically 50-50 highway vs city driving. 

Thanks in advance!

 

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My 5.3 averaged 15.5L/100km over the 100,000km that I owned it. 4x4, heavier LT tires, no air dam, 3.42 rear end. Probably 50/50 city and highway driving. How much have you driven? What are you getting for mileage on the highway? 18.0L/100km isn't wildly high if you factor in winter gas, idling, and city driving. The best I'd get in the summer was around 12.5 to 13L/100km over 650km. I don't think break in has much to do with anything. For the two trucks I've owned I didn't see much of a difference throughout the life of them. 

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Breakin has little to no affect. With the mods that you have, you will have to keep up with the mpg manually for a while to see what you "really" have. No way that I would go in and mess with the computer with a programmer right now, just in case there really is an issue and GM could use that to deny warranty! Do manual mpg calculations for at least 2000 miles then re-evaluate your results.  I have a 2018 6.2 Ultimate that has averaged around 21.5 mpg over its first 6000 miles. I do drive like grandpa most of the time  and try to avoid long warm-up idling/sitting idling etc but anyone that drives a new 6.2 has to "release the beast" every once in a while or their head will just explode!  Haha

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2 hours ago, Loafer987 said:

According to my cluster and GMC app I have averaged 16.1 l/100 km (14.6 mpg) since the truck was new.  Now according to my Drivvo app I am averaging around 18.0l/100km (13 mpg) which is also what my math comes out to when I calculate it myself after the past few fillups. 

Considering the season and your modifications, you are getting very decent fuel economy, imo.   Your larger diameter tires probably account for the discrepancy between your cluster and your app.   I went with larger and heavier tires on my last truck and decided the look wasn't worth the hit at the pump. Your added unsprung weight from increasing tire size won't get you more places than the stock sized version of the same tire.  Also, take some of the mileage claims on this forum with a "grain of salt".  Members on this forum represent a very diverse set of driving styles and road configurations. Both "highway" and "city driving" can look different depending on where you live.

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What's your average speed been?  The lower your average speed, the worse the mileage generally speaking.

 

I bought a new van once (2006) whose motor was very tight.  It did not get very good mileage and despite have sixty more horsepower than our identical older vehicle, did not feel like it had that much extra power.  Then magically at 9000 miles everything changed and she was a different animal.  Mileage greatly improved as did the power.

 

Start a spreadsheet if you are so inclined and record everything.  How much you pump in, fuel used, engine hours, etc. everything you can record.  This way you can spot trends and improvements.

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You did several things right off the bat that effect mileage. Lift, heavier / larger tires, and removing the air dam. Without a comparison from stock it is really hard to tell if you have an issue. Also remember the larger tires will cause your speedometer to be off, causing your on board calculations to be off.

 

285/60r20 , is that an LT, or P rated tire? What is the weight difference between those and stock? 

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I'm in a similar boat as the OP.  Your 285/60r20 is a 33.5" LT tire, that's your biggest problem (what kind?).  I noticed a 2mpg drop when I did aggressive tires.

 

Your toneau cover is actually hurting mileage too.  It's a myth it increases mileage.  

 

Putting your air dam back on would help a very small amount. 

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I have a stock truck and I have manually tracked over 6,000 miles of fuel economy. My  hand calculations are regularly 1.5 mpg less than what the display claims. It’s as if GM purposely programmed the computer to give readings of 1.5 mpg better than reality. It is way too consistent to be “variable” or an “acceptable level of accuracy.” I recall a YouTube video, I think TFL, that was comparing the GM/Ford/Ram, and they too had a similar discrepancy. 

 

Also, I have averaged 16.3 mpg in those 6,000 miles, hand calculated. Mine is 50/50ish highway/street. With your bigger tires, lift, air dam removal I think your not too far off. 

 

I wasnt happy with my MPG either in the beginning. I filled it up, drove 80ish miles on the freeway (40 each direction) at 55mph and immediately filled it up. It was maybe 1,000 feet of street driving, and I got 20.5 mpg.

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The tires are LT. Goodyear wrangler duratracs. I was a bit disappointed they sold them to us as "stock" as they didnt calibrate the truck computer/speedometer to match them. They also rub on the upper control arms so now there is no paint on the spot they rub on. But that's an entirely different issue. I run them around 36 psi at 0*C and was thinking over inflating them a bit might help a tiny bit too. Air dam will go back on as soon as I can find the time and the 70 screws that the dealer handed us in a bag when they took it off.

My big issue is the 3 co-workers I have with the same truck (2 2016s and a 2017) are getting way better mileage than me. But they also all have 20000-40000 kms on their vehicles and theirs are basically stock compared to mine. I'll continue to keep an eye on things and report back if I find anything substantial. I'm always open to ideas in the mean time as well. Thanks!

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My tires are 275/65r18 Duratracs, so a 32.1" tire vs your 33.5.  Mine is also a P rated tire that weighs 46lbs to your 59lbs. I lost about 2mpg.  I could see yours being 3mpg (however thanlt converts, i went from 18mpg to 16)

 

I didn't notice any drop with my 1.4" level/lift though.  

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My tires are 275/65r18 Duratracs, so a 32.1" tire vs your 33.5.  Mine is also a P rated tire that weighs 46lbs to your 59lbs. I lost about 2mpg.  I could see yours being 3mpg (however thanlt converts, i went from 18mpg to 16)
 
I didn't notice any drop with my 1.4" level/lift though.  
Excellent info here thanks. I didnt realize the tire weight difference was so drastic. Being that most of my mileage will be put on in the winter I'll try some winter tires/rims if I can find a good deal and see how the mileage looks. Went to put the air dam back on this afternoon and I cant find the stupid screws that they gave me when they took it off. Guess I'll have to wait till after christmas.

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gal3.jpg

It's a myth it's a myth. 

12 hours ago, Yondu said:

I'm in a similar boat as the OP.  Your 285/60r20 is a 33.5" LT tire, that's your biggest problem (what kind?).  I noticed a 2mpg drop when I did aggressive tires.

 

Your toneau cover is actually hurting mileage too.  It's a myth it increases mileage.  

 

Putting your air dam back on would help a very small amount. 

:lurk:

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The air dam might be 1/2 mile a gallon,the tires are probably 2 to 3 miles to the gallon,the level kit is 0 miles to the gallon.I did all those mods in different stages and that’s my specs.My truck hates cold weather and winter blend fuel,that’s another 2 to 3 miles to the gallon.When the temp hits about 55 degrees my mileage starts going back up.

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33 minutes ago, Grumpy Bear said:

 

gal3.jpg

It's a myth it's a myth. 

:lurk:

Ok, ok!  Tonneau cover works if you install a enormous gurney flap :). I think in essence the tailgate (with no cover) is like a gurney flap.  

 

 

I've never tested it, but it has been proven to decrease mileage.  As Ron Burgundy said...it's science.  

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I also found this to be a good scientific read but unfortunately only tests older trucks and it's a very old study now.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.agricover.com/downloads/pdf/2007_windtunnel.pdf&ved=2ahUKEwiz0eiJ27nfAhVizlQKHXVJBSg4FBAWMAB6BAgCEAE&usg=AOvVaw3G3799ApMcBgQEbTxbOpp6

I'd love to see someone put to bed the myth but there's contradicting info everywhere. Mythbusters made a basic attempt at it and the results were basically within margin of error. So my take from it is, tonneau covers can save you fuel IF the truck can benefit from the lower air drag due to poor design or mods that change the drag of the vehicle. (Like a lift)

Honestly the 2 best tanks I've had since I bought the truck were both using premium fuel. 91 octane made a bigger difference in fuel economy than any other fuel consumption "improving" mod so far. But more testing needs to be done...

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