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Duramax Fifth Generation: 2011-2016 (LML) Discuss the fifth generation (2011-2016) of the 6.6L Duramax diesel engine & associated components. Engine related discussion ONLY.

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Old 01-17-2019, 03:20 PM   #11 (permalink)
SyntheticAlex
 
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On my 2015 i was seeing 25 a gallon driving an hour to work every day at 60mph. PPEI ez lynk.
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Old 01-27-2019, 12:30 PM   #12 (permalink)
windy
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I have found that when towing, the wind is as big or bigger variable as the hills. Going 62-65 on interstate (towing a fiver) I get around 12, maybe 12.5 on perfectly level roads and no wind. With a 25-40 mph head wind (or even at a 45 degree angle) it drops to 7.5-8.5.

I suppose if I was on a 6 or 7% uphill, it would be around 7mpg also, but those grades don't last over 5-10 miles before going back down.

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Old 01-27-2019, 05:54 PM   #13 (permalink)
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Quote:
Originally Posted by windy View Post
I have found that when towing, the wind is as big or bigger variable as the hills. Going 62-65 on interstate (towing a fiver) I get around 12, maybe 12.5 on perfectly level roads and no wind. With a 25-40 mph head wind (or even at a 45 degree angle) it drops to 7.5-8.5.

I suppose if I was on a 6 or 7% uphill, it would be around 7mpg also, but those grades don't last over 5-10 miles before going back down.
wind resistance is a square law relationship to work needed. hills are a linear relationship to work needed. So, yes, wind can easily be a bigger impact than hills or changing speed (same linear relationship to the work needed)

On flat ground and constant speed, weight has almost no effect on fuel economy. In theory, none, because you only need to do work to change speed or elevation, but with additional weight usually comes along higher rolling resistance from wheels, bearings etc. and areodynamics which have some negative effects on mileage.
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Old 01-28-2019, 01:08 PM   #14 (permalink)
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oops, I made a mistake. the work needed to change altitude assuming a constant speed is a linear function W=FxD). Work needed to accelerate is a square law function (1/2mVsquared)
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Old 01-28-2019, 05:59 PM   #15 (permalink)
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So many factors will change your fuel mileage. Here is what I have found out. SPEED is the determining factor in what kind of mileage you get. Slow it down to sip, speed up to drink. My truck came factory with the 18 inch wheels and would get 19-21 on the highway ( all things being equal) at 80 mph. I swapped to factory 20 inch wheels, that same 80 is now 16-17. Slow down to 70 with the 20s and it got 21-23. Best mileage it ever got was pulling a jet ski on flat ground at 60 mph, 26.4 high with an avg of 25.3. I made the drive from Pensacola to Atlanta on less than a quarter tank. Regens kill mileage, speed kills mileage, bigger tires and wheels kill mileage. You can make em sip, you just gotta slow down.
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Old 01-29-2019, 07:30 AM   #16 (permalink)
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BigMike,,did you recalibrate the speedo fo the 20's?
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Old 01-29-2019, 08:04 AM   #17 (permalink)
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I did not. The hand math shows less than a quarter inch difference between tire sizes.
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Old 01-29-2019, 10:35 AM   #18 (permalink)
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Quote:
but those grades don't last over 5-10 miles before going back down.
You must not live in Phoenix at 1400 ft elevation and have to go up hill to 8000 ft just to get out of the Valley.

65 pulling is the general speed, mainly because the tires on the trailer are not rated for more but that also is the best fuel mileage when pulling, except climbing to Flagstaff or the Rim. I can expect 4-7 on those climbs.
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Old 01-29-2019, 11:02 AM   #19 (permalink)
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Bigmike, here’s a tire calculator link below where you can enter your factory size and your new size to see the mph differences between the two sizes.
1/4” would be pretty close to being 2 mph difference being speedo showing 65 but your actually going about 67.

Which also means your miles driven showing on your speedo will be less than what you actually have driven.
If you don’t add in the miles driven difference figuring mpg, your mpg will be a little lower.
This is one reason your seeing a little lower mpg, not having your speedo recalibrated
https://www.discounttire.com/learn/tire-size-calculator

Lynn, I’m in Phx closer to 1200’, climbing the hills going to Flag and just about anywhere out of the valley takes a toll on mileage on the return trip back down the mileage increases, which you know, this makes for a little better all around mileage towing 13,500lbs.

Mines about 6-8 climbing round trip average maybe 10 providing I don’t have much head wind going back to Phx.
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Old 01-29-2019, 12:24 PM   #20 (permalink)
BigMike8504
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2/10s of 1 mile per hour. Instead of 65 at 65, Im going 64.8 at 65. That being said, me being a tire guy, I know that those numbers are just rough estimates. Not all tires run true to size, some are bigger and some smaller, and they all loose just under a half inch of tread between new and worn out. I didnt take the time to put a string around each tire and figure out the avg circumference of the set ( although the fronts wouldnt matter as the speed for the speedo is measured on the rear tires, until they are rotated, anyway) of the old ones verses the new ones. That is really how to figure out the difference and it would have to be done by hand. I said all that to say, in my opinion, unless ones moves over an inch in tire size, the speedometer and fuel economy difference are minute. Per my edge monitor, the engine is turning 30 rpm less with the 20's than it did with 18s, at 80 mph.

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