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Duramax Sixth Generation: 2017+ (L5P) Discuss the sixth generation (2017+) of the 6.6L Duramax diesel engine & associated components. Engine related discussion ONLY.

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Old 12-11-2018, 12:34 AM   #41 (permalink)
RDP
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2018 GMC 3500HD with 100 miles, mostly short 2-20 miles trips till spring comes.
15 MPG

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Old 12-11-2018, 09:21 AM   #42 (permalink)
ata9977
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Quote:
Originally Posted by RDP View Post
2018 GMC 3500HD with 100 miles, mostly short 2-20 miles trips till spring comes.
15 MPG
I am in that same boat...

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Old 12-14-2018, 08:16 AM   #43 (permalink)
averey0
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I've gotten as high as 20 or 21 all hwy on fairly flat ground. 19 with mostly hwy but a little stop and go and or slight hills. Now though with winter fuel i'm down to 17-18 in these same conditions. The current tank i'm on currently sits at 15.8 with mostly around town driving and maybe 20% hwy.
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Old 12-18-2018, 06:24 PM   #44 (permalink)
deminimis
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Hey Kids. Some good numbers here. I recently sold my Ram 5500 and I'm thinking it's time to return to the flock (once I sell my new Tacoma TRD Pro). Time to just have one vehicle for me. Please keep those 3500 dually numbers coming!
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Old 12-20-2018, 10:04 AM   #45 (permalink)
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My road trip last August from Loveland. Co to Marshall, Mn and back.
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Old 01-03-2019, 10:07 PM   #46 (permalink)
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Got 17.5 mpg on the way home from the dealer but havent hit more then 15 mpg since. About 11.5 after the lift . Wonder if these new tunes will increase mpg enough to justify spending $4500 or are the just for hp/tq gains
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Old 01-04-2019, 08:28 PM   #47 (permalink)
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Roughly 1500mi on truck; cruise control set at 80-85 on long stretches of an 1100mi trip, I saw 15.5-15.7mpg.
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Old 01-04-2019, 10:23 PM   #48 (permalink)
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Sometimes speed is more important then fuel economy
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Old 01-05-2019, 09:14 AM   #49 (permalink)
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Agree redwngr, another form of, "you gotta pay if you want to play"

With the poor aerodynamic form of big pickups, increased speed kills fuel economy due to air drag so unless you always plan your travels to run with a big tail wind it will cost you. Every time you double your speed on a flat road it takes approximately 8X more horsepower. Normally doubling the rate at which work is done would require only twice the horsepower but because the air resistance follows approximately a square law response you end up with 4 times the air resistance at twice the speed coupled with the need to increase horsepower to do the same work at a faster rate you end up wit the basic 8X equation.

Or put simply, it if takes 10 horsepower to travel 40 MPH your horsepower demand goes up to around 80 horsepower at 80 MPH and you will burn more fuel to generate this power.

So why don't you get 8X the fuel economy at half the speed? The 8X approximation is a pretty close rule of thumb at high speed but at lower speeds you are also overcoming additional drivetrain losses, rolling resistance, any road grade, etc. as you increase speed and the powertrain is designed and geared to produce best efficiency under typical operating conditions. So even at lower speed you aren't going to be challenging a prius in the fuel economy race but at 52-55 MPH you should get in the low 20 MPG range which might be important if you are very low on fuel many miles from the nearest station. On the other hand if you remove the speed governor from the programming and get the truck into the 110 MPH range then you probably will be in the 3-4 MPG range or about what you would get running the truck fully loaded up the Ike with the engine RPM at peak HP.
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Old 02-13-2019, 05:58 PM   #50 (permalink)
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So far.........

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