2020 GMC AT4 Duramax fuel consumption

Jgreene11 · October 1, 2020

So I have 2000k miles on my Duramax and have been loving the engine. I drove it home initially from Oregon to Idaho highway mileage averaged close to 30mpgs and was ecstatic.
Just took a trip to Oregon coast and towed fairly light travel trailer (5000lbs) about 500 miles, mostly highway. I averaged about 10.5mpgs. Kept reseting the miles cause I thought this was incorrect. It was windy, and I wasn’t super heavy on throttle.
Way home, wind was light, driving conditions perfect and still averaged 11mpgs. I was expecting much better mileage. Any suggestions on why my mileage was so poor??

jeepincbum11 · October 1, 2020

14 hours ago, Jgreene11 said:

So I have 2000k miles on my Duramax and have been loving the engine.

You have 2 million miles on your 2020 truck? Did i read that right? or do you mean 2k miles or is it 2000 kilometer miles? HaHa.

As for the towing were you in trailer tow mode? i found i got better mileage when i wasn't it tow mode.

Jgreene11 · October 1, 2020

10 minutes ago, jeepincbum11 said:

You have 2 million miles on your 2020 truck? Did i read that right? or do you mean 2k miles or is it 2000 kilometer miles? HaHa.

As for the towing were you in trailer tow mode? i found i got better mileage when i wasn't it tow mode.

Haha! Thanks for correction. Yes, 2k miles. I was in trailer tow mode. I’ll try it next time without

newdude · October 1, 2020

14 hours ago, Jgreene11 said:

So I have 2000k miles on my Duramax and have been loving the engine. I drove it home initially from Oregon to Idaho highway mileage averaged close to 30mpgs and was ecstatic.
Just took a trip to Oregon coast and towed fairly light travel trailer (5000lbs) about 500 miles, mostly highway. I averaged about 10.5mpgs. Kept reseting the miles cause I thought this was incorrect. It was windy, and I wasn’t super heavy on throttle.
Way home, wind was light, driving conditions perfect and still averaged 11mpgs. I was expecting much better mileage. Any suggestions on why my mileage was so poor??

You aren't too far off. Plus with such low mileage on the engine, you might not see it improve for a few thousand more. Also being you have the AT4, the truck sits 2 inches higher than a regular one and you don't have the lower valance on the front bumper so 11mpg with an aerodynamic penalty is pretty good with a trailer and wind actually.

This guy did a good MPG loop.

Edited October 1, 2020 by newdude

redwngr · October 1, 2020

How fast are you pulling the trailer?

Flat deck or enclosed utility or camper or .../?

How tall is it?

ullose272 · October 2, 2020

So I have 2000k miles on my Duramax and have been loving the engine. I drove it home initially from Oregon to Idaho highway mileage averaged close to 30mpgs and was ecstatic.
Just took a trip to Oregon coast and towed fairly light travel trailer (5000lbs) about 500 miles, mostly highway. I averaged about 10.5mpgs. Kept reseting the miles cause I thought this was incorrect. It was windy, and I wasn’t super heavy on throttle.
Way home, wind was light, driving conditions perfect and still averaged 11mpgs. I was expecting much better mileage. Any suggestions on why my mileage was so poor??

I-84 at 80 mph?

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topgear1224 · October 2, 2020

Excessive friction as the engine breaks-in.

These 3.0 are TIGHT from the factory I regularly saw oil temps over 300 while not towing during break-in.

The GMCs have really poor aerodynamics above 55 compared to the Chevy.

Towing with a lift (At4), was your butt down?

this means instead of the air hitting the front of the truck it's actually hitting the bottom and acting like a huge sail pushing against the wind.

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Jgreene11 · October 2, 2020

5 hours ago, topgear1224 said:

Excessive friction as the engine breaks-in.

These 3.0 are TIGHT from the factory I regularly saw oil temps over 300 while not towing during break-in.

The GMCs have really poor aerodynamics above 55 compared to the Chevy.

Towing with a lift (At4), was your butt down?

this means instead of the air hitting the front of the truck it's actually hitting the bottom and acting like a huge sail pushing against the wind.

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YeH, but was down. A bit of a sag. Truck was all over the road in the wind.

Jgreene11 · October 2, 2020

6 hours ago, ullose272 said:

I-84 at 80 mph?

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Yeah, but more like 60-70 cause it was so windy!!

Jgreene11 · October 2, 2020

10 hours ago, redwngr said:

How fast are you pulling the trailer?

Flat deck or enclosed utility or camper or .../?

How tall is it?

60-70. Travel trailer, so maybe 10’ tall

topgear1224 · October 2, 2020

[emoji50] when it's windy and you have a tall trailer you need to drop your speed much more than that, think 55 or so.

That trailer would have no problem flipping your truck.

If you were doing 65-70 in windy conditions yes 10 mpg is what you should expect. The wind is whipping all over the place and not moving smoothly over the body, this requires much more power. Frontal area of the trailer is a big factor too.

Just be happy you don't have the gas engines, you would have been down to 7-8 mpg.

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Jgreene11 · October 2, 2020

7 minutes ago, topgear1224 said:

when it's windy and you have a tall trailer you need to drop your speed much more than that, think 55 or so.

That trailer would have no problem flipping your truck.

If you were doing 65-70 in windy conditions yes 10 mpg is what you should expect. The wind is whipping all over the place and not moving smoothly over the body, this requires much more power. Frontal area of the trailer is a big factor too.

Just be happy you don't have the gas engines, you would have been down to 7-8 mpg.

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Thanks for the info!

Supercup · October 2, 2020

10-11mpg towing a 5,000 box through the wind is pretty good.

I have 2020 AT4 and had 2019 AT4 before, both with 6.2L - towing my 24' Intech empty I am at 4,600 lbs +/- - at 60 - 70 I get 9-10 mpg. at 8,000 lbs I get 8.4 - 9.2 pending the number of hills.

So you are a little better, but not as much as you might have hoped. Regardless a very nice set up! Can't believe you got 30 mpg - I have never seen anything north of 21 in mine.

For comparison my 2019 over 10,500 miles averaged 14.5 mpg of mixed city and freeway driving, with about 2,500 miles of towing thrown in as well. Best tank was 21 and worst at 8.1 towing into the wind.

Jgreene11 · October 3, 2020

1 hour ago, Supercup said:

10-11mpg towing a 5,000 box through the wind is pretty good.

I have 2020 AT4 and had 2019 AT4 before, both with 6.2L - towing my 24' Intech empty I am at 4,600 lbs +/- - at 60 - 70 I get 9-10 mpg. at 8,000 lbs I get 8.4 - 9.2 pending the number of hills.

So you are a little better, but not as much as you might have hoped. Regardless a very nice set up! Can't believe you got 30 mpg - I have never seen anything north of 21 in mine.

For comparison my 2019 over 10,500 miles averaged 14.5 mpg of mixed city and freeway driving, with about 2,500 miles of towing thrown in as well. Best tank was 21 and worst at 8.1 towing into the wind.

Nice! Well thanks for the feedback. Been checking out your set up. Looks nice! I have Icons on order, with some Method 18x 9 and Nitto RG in 295/70/18. Hopefully in next 6 weeks

Supercup · October 3, 2020

13 hours ago, Jgreene11 said:

Nice! Well thanks for the feedback. Been checking out your set up. Looks nice! I have Icons on order, with some Method 18x 9 and Nitto RG in 295/70/18. Hopefully in next 6 weeks

I looked at Method Con6m us that what you have on order - or is it a Icon suspension with Methods? If you ordered the Con6 it does not look like you can order the -12 mm offset (4.5" of backspacing) 18 x 9's - only the 18 mm (5.5" backspacing). If you consider other Method options the -12mm with 4.5" of backspacing will give you more inboard clearance and better overall flexibility to fit larger tires. FYI stock AT4 18's are 26mm offset - so 18mm does pick up some inboard room - but the -12mm picks up a lot more.

I had the 18mm with 5.5" of back spacing - with Ready Lift 2" SST kit with UCA's on my 2019 - I had less than 5/8" of clearance to my 275/65/18 - I could not have fit a 295 - a 285 would have been very close. With stock upper control arms you pick up about 1/4", so a 295 likely would just fit.

I am using same wheels on my 2020 - when I added BDS 2.5" lift kit - the tie rod hit the wheel, so I had to add 1.25" spacer and lug extensions to make it work. Backing up at full lock I have some minor rubbing I have not sorted yet. So I am now going to swap to the -12 mm with 4.5" of backspacing so I can have more tire options and clear the tie rod without the spacer or lug extensions. I have Method NV's.

Good luck with you updates - should look good.

2019 with 18mm offset Method NV 18x9 - inboard clearnce stock Goodyear Duratrac - 275/65/18