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Old 07-22-2020, 12:04 PM   #1
PLM
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2020 duramax towing mileage

First, let me say that I have been towing a fifth wheel of some model for 17 years or so. So I'm not new to towing? I am currently towing a 2018 Carbon 347 fifth wheel with a 2020 GMC 2500 with the Duramax and the 10 speed transmission. The truck has about 13k miles on it now. The camper is about 13k pounds dry and 17k fully loaded. I figure I generally tow it at about 15k - 16k pounds? I have towed it to Ohio twice, Kansas City and Atlanta. I've tried towing several different ways. Tow/haul mode always. Locking out 10th, locking it out of 9th and 10th, leaving it in drive and just letting the transmission shift as it needs. I've tried towing it at speeds anywhere between 65 all the way up to 75mph. I've towed with and without the cruise control. With and without the exhaust brake. My average fuel economy is always about the same. About 8-9mpg (8.6mpg seems common). Is this the kinda economy (economy is probably the wrong word) others are seeing with this vehicle? I love the truck but my 2016 ram with the Cummins (fully deleted) did between 11.5- 12.5mpg setting the cruise about 68mph. Thanks in advance.
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Old 07-22-2020, 12:19 PM   #2
flybouy
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Do you know what gear ratio the differential is?
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Old 07-22-2020, 12:47 PM   #3
PLM
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The gear ratio is a 3.42
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Old 07-22-2020, 12:58 PM   #4
flybouy
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Thjat's a pretty tall gear ratio for towing. At 70+ what rpm are you turning?
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Old 07-22-2020, 01:04 PM   #5
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I like to run just at or a little under 70. At 70 in 10th it's about 1400rpm. 9th os about 1700 and 8th is about 1900 rpm. 7th takes it up to about 2400rpm. It tows best in 8th gear and with the 10spd I can I have the luxury of more ways to find its optimum range. Max torque is supposedly made about 1600rpm.
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Old 12-01-2020, 01:33 PM   #6
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Any fully deleted Diesel is most likely going to get better fuel mileage, whether towing or not, than a stock Diesel with all of the controls and emissions crap on it. Probably only way to increase fuel mileage will be to slooooooow down.
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Old 12-01-2020, 02:12 PM   #7
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In the real world, towing that RV (weight and cross section) will continually generate about 8.5 to 10 MPG, regardless of your gear ratio, make of engine or driving habits. Would you like to hear of RV'ers towing 44' Mobile Suites with their new Rams getting 14-15 MPG? RV'ers triple towing 5'vers with their new Fords getting 13 through the mountains? Facebook and the internet is full of them. Liars, liars, liars all of them. The baloney stories will start at happy hour in the RV parks and then continue around the campfire at night. LHM, it is imperative to bring out the hip boots some nights.
I will tell you that your mileage might improve ever so slightly by slowing down, but not proportionately like in a gas engine. And the idea of delete and all that? Not good for the environment and not good for the resale. Some people have a feeling for our environment and some...? Well, enough said.
Enjoy your ride. It is a nice truck and you're fortunate to have fared so well in life to afford one. Enjoy every mile and put the MPG on the back burner.
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Old 12-01-2020, 02:14 PM   #8
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My stock 16 Ram 2500 CTD 4x4 would get 11-12 mpg pulling 12-13K. 21mpg highway empty, 18mpg everyday running around. Sad day when I let it go with only 77K on the clock. Lowest depreciated vehicle I ever owned.
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Old 12-01-2020, 02:23 PM   #9
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Quote:
Originally Posted by notanlines View Post
In the real world, towing that RV (weight and cross section) will continually generate about 8.5 to 10 MPG, regardless of your gear ratio, make of engine or driving habits. Would you like to hear of RV'ers towing 44' Mobile Suites with their new Rams getting 14-15 MPG? RV'ers triple towing 5'vers with their new Fords getting 13 through the mountains? Facebook and the internet is full of them. Liars, liars, liars all of them. The baloney stories will start at happy hour in the RV parks and then continue around the campfire at night. LHM, it is imperative to bring out the hip boots some nights.
I will tell you that your mileage might improve ever so slightly by slowing down, but not proportionately like in a gas engine. And the idea of delete and all that? Not good for the environment and not good for the resale. Some people have a feeling for our environment and some...? Well, enough said.
Enjoy your ride. It is a nice truck and you're fortunate to have fared so well in life to afford one. Enjoy every mile and put the MPG on the back burner.
Jim, the way I heard it was that was driving at 85 with a 50 mph headwind. I guess it's because the hills are smaller out here in the east.
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Old 12-01-2020, 02:31 PM   #10
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I have a 2014 Chevy 3500 Duramax Diesel Dually, long bed, crew cab. As I've now towed 2 very different trailers over the years I have found that with my previous travel trailer (Outback), I averaged 10 mpg and not towing around 12 mpg.

The truck reached about 55,000 miles and something weird happened. I'm also now towing a 5000 pound heavier trailer. Towing I'm now getting 9 mpg but not towing I'm getting 18-20 mpg. Very strange, but also very consistent.

Now, my towing speed it 60 mph or the TRUE speed limit, which ever is lower. But just for giggles sake, I tested the waters a couple times this last Summer and dropped my towing speed to 50 mph for about 2 hours straight. I fueled up at the start and the end for a true measurement, and I got 12 mpg. The terrain was flat (and towing).

I've noticed when traveling the mountains (Kentucky, Tenessee, North Carolina, Virginia, West Virginia, I average 11 mph. Very weird. I tow in Tow Mode and Exhaust Brake always when towing, on any terrain.

If you are wanting better fuel efficiency, they try slowing down. By lowering my speed by 10 mph, I got about 5 mpg more. So your choice ... drive slower, takes longer to travel = better fuel mileage. Or, drive faster, takes less time to get there = poorer fuel mileage.

I think, as your truck gets older and more miles on it, it also will get better mileage. Mine astounds me when I average 19 or even 22 occasionally now. Yes, mine is also a daily driver.
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Old 12-01-2020, 03:59 PM   #11
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When I was towing with my LB7, depending on when I looked at the DIC it would be 4 mpg or 15 mpg or 99 mpg.

As to the OP why his mpg is that low, could be excess regens, they use a quite a bit from what I'm told. You have one extra injector that all it's doing is making the exhaust pipe hot.
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Old 12-01-2020, 04:36 PM   #12
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Figure that our Fuzion is about the same weight as your setup, and on a fairly flat cruise where I roll between 65 to 68 I get mid 9.'s mpg. I have seen as low as 6's once with a 20-30mph head wind and up to 11 on short runs under 60 mph when on secondary roads. I normally use tow mode, and will preemptively downshift manually in hills between 4-5-6 to keep from dropping too much speed or lugging a bit. Your L5P engine makes more hp ( 445 vs 395) and torque that my 2015 LML, so even with a reworked turbo configuration, it may take more fuel to make that HP.
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Old 12-06-2020, 09:01 AM   #13
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I tow wiht an older GMC... LBZ stock and stock tire size. Towing a 34 foot Alpine.. at 62 mph my RPM is always around 1450 and I average 10.5 to 13.5 mpg and have been for the last 7 years...

Id get into a gear that drops that RPM down a bit.. I Think...

Yes its a newer 6.6 Duramax with 10 speed and your gear ratio is different than mine.. Mine is 3.73 and Allison 6 speed

I two in tow haul mode with cruise on except up and down grades...and were I live those are 8% or more at times

my MPG is hand calculated and is always am over 10.5 unless Im towing with a head wind
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Old 12-06-2020, 12:05 PM   #14
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For quite a while recently the temp gauge had been all over the place, never overheating but a lot of the time never getting up to where it had always been. Finally got a check engine light and it turned into it to be a code for the thermostats. Ordered new ones, there are two under the same housing on the Duramax engines, and installed them. We are getting much better fuel mileage now, back up to about where we used to be at. All this to say, it doesn’t take much to mess up fuel mileage, and if things suddenly change lower, it might be worth it to get it checked, even before a check engine light comes on.
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Old 12-06-2020, 01:05 PM   #15
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I have a 2020 3500 DRW I tow another brand (sold the fuzion this past spring). Triple axle GVW 20k Just did a trip from CT to southern Florida and averaged 8.9 MPG So yeah your numbers are right
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Old 12-07-2020, 06:22 AM   #16
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Also it’s a good idea to remove the radiator stack top plastic cover at least once a year and blow out the area (stack) between radiator and cooler ... this will cause you issues with Allison running hot on a good climb... I do this once a year and makes huge difference in my Allison towing temps
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Old 09-19-2021, 08:03 AM   #17
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Just love the ride.

I tow a 43’ triple axle Fuzion with SxS loaded to about 18,000 +/- with a 2020 GMC 1-ton Denali Duramax 10-speed. 8-10 mph is my average including plenty of mountainous roads. Everyone I’ve talked to with a similar setup gets about the same mileage. Slow down a little and enjoy the scenery. I bought the truck to handle the load, not for the mileage. My best unladen mileage has been a remarkable 25.5, but averages 18-19. Under a heavy load it will naturally be cut nearly in half. Really don’t care because the truck is perfect in every other way. I have discovered that old RV’ers are much like old fishermen around the campfire… “should have seen the one that got away…”
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Old 09-19-2021, 09:05 AM   #18
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I have the same 2020 GMC Duramax 10 speed. I tow a Cougar 27SAB. California speed limits are 55 at which I can get around 11 to 12 on flat highway with no winds. In reality with winds and varying grades at speeds between 60 and 70 I get a consistent 10.
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Old 09-19-2021, 07:05 PM   #19
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My lml 2500 with the same load pulling as the 3500 L5P Average 10 mpg . The l5P average 8.4 2000 miles towing with gvwr 23000 lbs . I look at more power more fuel and DEF.
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Old 09-20-2021, 01:32 AM   #20
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We have a 2020 3500 DRW. We have averaged 12mpg traveling from NJ to the finger lakes area of NY which included some towing through elevation changes. Cruise control set to 60mph, exhaust brake on. We haven’t weighed our rig yet. Truck front tires 70psi, rear 80psi. Trailer tires at 110psi.
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