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Old 04-18-2017, 10:11 AM   #15
"Formerly Diplomat Don"
 
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"krzy parents".......You answered your own question. At 70 mph, you're doing good to get in the 6's. As you get above 60, the mpg numbers drop exponentially. Your 60-62 mpg is what I'm seeing and I do many things to conserve fuel.

I don't baby my coach, especially on grades, but I do other things to help my mpg. I run the VMSpc program that gives me instant rolling mpg, HP and torque numbers that are estimated. To leave California, we either end up driving through the desert or up Hwy 99. Some days we get a tailwind in the desert and some days we get a headwind. By watching the rolling mpg, I can see when there is a headwind (not noticeable by looking out the window) or there is a slight grade that's not really noticeable. When these occur, I'll often change my speed a couple of miles an hour and pick up some mpg. Just the opposite.....if I see my mpg going up because of wind or grade, I'll throttle up some. The sweet spot on my coach is 62-63.

I never liked the drive by wire feel of these DP motor homes. You just can't feel if you're giving it to much throttle when accelerating. For that reason, when entering on ramps, I accelerate to about 40 mph and then engage the cruise control. Then I bump up the throttle, via the cruse control, every 3-4 seconds. This allows me to accelerate without dumping unnecessary fuel into the engine. When exiting off ramps, I'll shut off the cruise and engine brake and coast off the ramp. I still exit at a reasonable speed as not to block traffic.

Lastly, I use the cruise control all the time, even in light traffic. I can "thumb" up or down my speed very quickly with the cruise control. I change my air cleaner every 18 months and run OptiLube for the extra lubricity sometimes missing in diesel fuel. Even those these last two items aren't measurable, I feel my coach always runs at an optimum level.

All of these may sound trivial, but I've incorporated them into my driving habits to increase my mpg.

Then.......there are just some days, like late in the evening....I just throttle the beast up to get to my destination.
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Old 04-18-2017, 10:29 AM   #16
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My '02 DSDP with the ISC 350 would get 7.5 to 8 mpg. I just used 7 for figuring trips. My present rig with the ISX has gotten 7.6 over it's lifetime. I run right at 60 - 62 when it's legal to do so (outside of CA). 60 mph is 1350 rpm, and Cummins mileage calculator shows I should run 1370 for best mileage. I still use 7 mpg for figuring trips as my AquaHot and generator use from the same tank naturally.
I've also used OptiLube in the Dutch Star but kind of wonder about it's effects on my DPF since I don't have the DEF injection in the present rig.
But if you're really worried about fuel mileage, get an electric car and stay in motels.
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Old 04-18-2017, 10:51 AM   #17
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Quote:
Originally Posted by krzy parents View Post
Thanks for all the comments. There are variation factors for sure with terrain and wind. I really don't notice much difference whether pulling the tow dolly or not. However, I did find the 'sweet spot' on my 400 cummins engine. If I cut my speed down to about 60-62 mph the engine just shifts into 6th gear and then I do get an average of 7.9-8 mpg. If I kick it up to 70 mph then I go back to 6-6.5 mpg. So best to slow down and enjoy ride and just let the fast pace traffic pass me by!
Ours is 63 mph @ 1850 rpm. Front tires @ 110 psi & rear duallys @ 105 psi.
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Old 04-19-2017, 04:05 PM   #18
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Read what Cummins says for better MPG...
Attached Files
File Type: pdf cummins_secrets_of_better_fuel_economy.pdf (964.6 KB, 750 views)
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Old 04-19-2017, 04:33 PM   #19
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Read what Cummins says for better MPG...
Notice that it says the break in period for tires is 35,000 to 50,000 miles. Most of us have to change tires due to age long before they are even broken in.
And the the difference between the best and worst drives can be as much as 30%. So, am I a great drive or a poor driver? That's why these mileage discussions don't mean much, too much variation between rigs and drivers.
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Old 04-19-2017, 07:47 PM   #20
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6.5 avg w/450 ISM Cummins, Allison 4000, 42K actual weight towing 4700# vehicle. There are some generator hours and Oasis hours in there which throw the average off occasionally, so it could be a whisker higher toward 7mpg. It went up slightly after a couple tanks of diesel injector cleaner.
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Old 04-19-2017, 09:31 PM   #21
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The C9 Cat on my MH has been averaging 7.5 mpg for the last 35000 miles. Scan Gauge D is very helpful in seeing how you are really driving.
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Old 04-20-2017, 06:18 AM   #22
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ISL 400...

We drive 65mph with JEEP Unlimited in tow...our mileage ranges between 8.2 and 9.2 depending on how much maneuvering it takes to get on the highway. I admit...the 8.2 range is most common.

Hoping the solar panels I installed don't bring down the mileage any...
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Old 04-20-2017, 06:20 AM   #23
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Charlie, do you know how to wire the panels to motor

may increase your MPG
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Old 04-20-2017, 07:11 AM   #24
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In general, air resistance is proportional to the square of your speed. The difference b/t 65 and 70 in my rig translates to about 200 rpm (~1700 to ~1900). In my case, better fuel mileage translates to driving a bit slower (65 mph).
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Old 04-20-2017, 07:55 AM   #25
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I still got 6 to 7 mpg with the engine turned off and coasting down hill LOL
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Old 04-20-2017, 08:14 AM   #26
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Charlie, do you know how to wire the panels to motor

may increase your MPG

Hmmm...now you're thinkin'

The first Hybrid Ventana...solar powered. Maybe electric motor drives on the tag that go into effect at a certain speed after the tag axle locks... wow!

I need more batteries....
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Old 04-20-2017, 09:58 AM   #27
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Since I had my 2006 (engine) 500 ISM I averaged 5.6 mpg with a little generator and Hydrohot use thrown in. This spring coming from Texas to VT I averaged 6.6, 6.7 and 6.8 on my last three tanks. On these last three tanks there was very little wind in any direction which may have made a big difference. Also summer fuel and warmer temperatures. Finally I slowed down form 63 mph to 58 mph @1380 rpm. My engine shifts to 6th gear at 58. Not sure what the real change is but over 1mpg makes a big difference! This is all towing a lifted Jeep Wrangler Unlimited with 37" tires that gets 15 mpg by itself.
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Old 04-22-2017, 05:39 AM   #28
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Ours is 63 mph @ 1850 rpm. Front tires @ 110 psi & rear duallys @ 105 psi.

my DS 3623 350HP also will run about 63 - 1580rpm, the pencil calc shows about 7mpg when the computer shows about 6. Now 63mph is GPS speed and the speedo shows about 70. The speedo is way off. I think everyone might be checking the speedo accuracy.

My questions are; What MPG are you getting with these tires pressures? What is your coach model? At about 15 pounds high pressure that you are using I wonder if you feel your ride is rough? MY pressure is front 90, back-100 and some of bridge expansion joint sound like a shot gun going off in my ear.

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