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Old 05-16-2014, 10:08 AM   #15
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here is a article from cummins at fmca that is pretty good

Torque Specs On Cummins 400 | Tricia Joy

My 42' dynasty with ISL 400 likes about 63 at 1700 rpms for the best mpg. I have to go over 56 to make torque converter/ allison most efficient 6th gear no searching. It does not seem to make much difference if 1600 or 1700 rpm but max torque is way down at something like 1250 but I can not run that slow.

Sheri & Don 2003 Dynasty 42" regal ISL 400
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Lets go we got it all loaded.
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Old 05-16-2014, 10:43 AM   #16
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Originally Posted by docj View Post
This paper by Caterpillar discusses how road speed, RV weight, wind speed, etc, affect engine power usage and, therefore, mpg for diesel-powered MHs. It doesn't matter if your engine is a CAT or a Cummins, the relationships are the same: Understanding RV/Coach Performance
That's a great reference source. For the Cummins crowd, here's a very similar resource: http://cumminsengines.com/uploads/do...el_economy.pdf

Pretty much all the same stuff - biggest difference is a tendency to favor red over yellow and black for page headers.

Take care,

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Old 05-16-2014, 11:09 AM   #17
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Put me in the 62-67 crowd. Cummins ISB, Allison 5 speed, 20k lb Dp. Total miles to total fuel =11.1mpg, almost always with toad.. At 55-58mph, tends to lose momentum on small hills, and to search for correct gear, mileage drops. At 62+ maintains speed within 1-2mph, seems to hold top gear well. Although I'm seldom at 68-70, the mileage starts to drop, about 1/2mpg. If I'm above 70 something is chasing me...
FWIW strong headwinds seem to have the most effect on fuel use..
50-60 in 20+ headwind resulted in a 9.2mpg leg.
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Old 05-17-2014, 05:11 AM   #18
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Here is great info from Caterpillar about speed,HP and MPG.

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Old 05-17-2014, 05:25 AM   #19
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The problem with trying to drive based on published spec's is if you have a gasser that's a few years old and are pouring Ethanol enriched gasoline into it, those numbers go out the window.
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).
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Old 05-17-2014, 06:00 AM   #20
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10,000 miles at 7 = .41 per mile
9= .53 per mile
at 3.70 per gal. go from there and make up your own mind
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Old 05-17-2014, 07:45 AM   #21
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The biggest factors are weather, wind, road conditions, city or hwy driving. Last summer did a 5700+ trip to 13 Western States in all conditions. Averaged 9.78 mpg for the trip. Ran both roof AC and cab AC in the hot weather and no AC or Gen in normal weather. Went through the mountains a few times and drove through Kansas in head winds. Found that the best for me was around 60 for ease of driving and looking at scenery. The faster you drive the more gas you use. I don't calculate anymore because it is what it is.

Sit back and enjoy the ride.
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Old 05-21-2014, 02:05 PM   #22
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We do 55 mph when it's windy and 62 mph (max) when its not with our 8.1 liter gasser. Enjoy the ride and slow down!!!
Rick and Lynda Smith
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Old 05-21-2014, 02:21 PM   #23
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I'm speaking of a diesel pusher, I have no gasser experience. When I was still living/working in Ca and frequently drove the exact same route to/from my ranch in southern NM, I would experiment driving the whole trip at 60, next time all at 65, then 70. I would never drive under 60 on I-10 you'd get run off the road. But overall it never made more than .4 MPG difference whether I drove 60, 65 or 70. Now that I'm retired, I just drive 62-65 and ignore the MPG, it is what it is; for my Cat C7 about 7.5 - 7.75 overall avg. As others have said, there are many variables involved, some of them just as important as the MPH.
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Old 05-21-2014, 02:39 PM   #24
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I noticed when I drive over 65 I loose a lot in handling so I always keep it under that. We left early in the morning on a Monday because my wife had to go to work so while she was sleeping I pushed it on up to about 70-75 and all that did was give me white knuckles. 62 is my MPH and I hope the MPG reflects.
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Old 05-23-2014, 06:21 AM   #25
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We plan for 8 mpg and 65 mph and are generally pretty close. Long stretches of 2 track (like across Mi's UP) might have us down to 60 long enough to affect our mileage, but it's not going to be much. Maybe 1/2 mpg? I can't even imagine driving 55 long/often enough to see what that might do.
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Old 05-23-2014, 08:58 AM   #26
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My motorhome is 12' 8" high and 8' 6" wide. That is the equivalent of stacking three 4' X 8' sheets of plywood on top of each other and pushing them down the highway. That is a lot of wind resistance. And the drag increases logarithmically which means that as you increase speed the drag doubles. I have actually driven an average of 45 mph in the mountains and gotten better mileage than driving at 65 mph on the flat. Speed makes a tremendous difference. I also drove the Natchez Trace which limits your speed to 50 mph and is flat. I got the best mileage that I have ever gotten. I agree with other posters though. You cannot drive at the best speed for the highest mileage on the highway because you will impede the other traffic. My Natchez Trace trip gave me mileage that was about 1.5 mpg higher than my average. Of course your actual numbers will vary based on the number of gears in your transmission, your loaded weight, weather or not you are towing, etc. My math says that the difference between 6.7 mpg and 8.0 mpg at $3.70 per gallon is 16.4% in cost per mile. This means to me that you do want to drive conservatively and keep your speed reasonable (don't drive 70 to 75 like I see some RVers do). It is worth it and is safer too.
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Old 05-23-2014, 09:20 AM   #27
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Originally Posted by SissyBoyBob View Post
Lets say that 8 mpg was the average, just for this conversation. How much would that change by driving 65 MPH as opposed to 55 or 75mph. What is the perfect speed to cruise at in order to get the best MPG? Would speed change it more than 1 mpg, or does it really not make that much difference at speeds most of us drive?
I cruise at 60 MPH and get 8 MPG.
IMO, (and experience), nothing, (short of a miracle), will improve the mileage of an 8 MPG RV by 1 MPG.
'96 Safari, 132kmiles, 8 MPG, (averaged over 105 miles)
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Old 05-24-2014, 06:24 AM   #28
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MPG, is what it is. You can't own one of these behemoth's and worry all the time about the miles per gallon. My experience in owning a lot of different types of vehicles through the years, is that they all have a sweet spot where they seem to run the smoothest & best. Mine is 65mph on the interstate's, and slower on back roads. It works for me.

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