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Old 11-06-2021, 12:50 PM   #1
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Cummins ISL 400 estimated mileage

I have a 2005 tiffin with Cummins isl 400. Pushing 32,000 lbs. with C-RV tow.
I am consistently getting right at 5.6 mpg. Running at 65 mph at 1700 rpm. Not heavy footed by any means. Everyone seems to be getting better fuel mileage. Any suggestions as to what i could do better. 97k on engine.
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Old 11-06-2021, 12:55 PM   #2
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Yeah, that's terrible mileage for your weight!

Only thing I can think of is slowing down 4 or 5mph.
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Old 11-06-2021, 12:59 PM   #3
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Ya, if driven reasonably, you should be in the 7.5- 8 range.


That engine is prone to cracking the exhaust manifold. If cracked, boost is down.


Easy to check with a flashlight-- look for BLACK from exhaust where it shouldn't be.
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Old 11-06-2021, 01:01 PM   #4
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Agree with the previous two posts.


Exactly how are you calculating your mileage?
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Old 11-06-2021, 02:11 PM   #5
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We're almost identically configured, except our weight is slightly higher. We even tow a CR-V. We're seeing 7-7.5 mpg at 62 MPH. (I like to have 3 MPH in hand because the CR-V's tow speed limit is 65 per the owner's manual.) I have tried speeds from 55 to 65 and it does not to seem to make much difference. I think we're turning about 1600 RPM at 62, but I'll check when I drive it next (it's due for an exercise drive) and report back.

We just broke 100K miles on the coach.

Do you have a Scan Gauge or something similar? We could compare numbers for RPM, boost, engine temperature, and so on.

Finally, where are you driving? The mileage I quote above is highway. If I drive, say, around Tampa I can easily see 5.5-6 mpg on a *good* day...
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Old 11-06-2021, 07:46 PM   #6
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Some info from Cummins on fuel mileage.
Attached Files
File Type: pdf cummins_secrets_of_better_fuel_economy.pdf (964.6 KB, 43 views)
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Old 11-06-2021, 09:07 PM   #7
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I had a 2005 36' Monaco Diplomat that weighed exactly the same, 32K. I towed various vehicles, including a Silverado. Our usual speed was 63mph. On a 7000 mile trip, coast to coast, filling at the same station at the start and finish and at the same pump, we got 8.9 mpg for the trip. The calculation was done with pen and paper.

With that said, I added a Source Engineering engine fan, which returns 25HP to the engine. It was a more efficiently designed fan. I also installed a Digi CR engine chip which produced an additional 52HP. The coach was a rocket and when carefully driven, could hit the high 9's on flat drives. We live out west and lot of our driving was mountainous.

Even without the improvements, we routinely saw low to mid 8 mpg. Something's not right on yours. I long for the mileage of the Diplomat, my current coach struggles to get 6.8 mpg.
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Old 11-06-2021, 09:49 PM   #8
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Do you run your genset when traveling? Our MH has a GVWR of 31K and a ISC 530 hp engine. When I do not use the genset I average 8 MPG, when I use the genset mileage averages 6.5-7 MPG. That is trying to maintain 62-65 MPH.
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Old 11-06-2021, 09:58 PM   #9
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8.8 mpg, 35k lbs including pulling Accord, I drive 55-60 mph. '15 380hp ISL 8.9L
Hold your foot steady, avoid headwinds, don't drive in packs, anticipate traffic and lights.
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Old 11-07-2021, 01:44 AM   #10
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Quote:
Originally Posted by wolfe10 View Post
Ya, if driven reasonably, you should be in the 7.5- 8 range.


That engine is prone to cracking the exhaust manifold. If cracked, boost is down.


Easy to check with a flashlight-- look for BLACK from exhaust where it shouldn't be.
Will take a look at the manifold, thanks for the info
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Old 11-07-2021, 01:48 AM   #11
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Originally Posted by Ljwt330 View Post
Agree with the previous two posts.


Exactly how are you calculating your mileage?
Pen and paper. Fill tank to same level at each fueling.
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Old 11-07-2021, 05:17 AM   #12
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Do you know for sure that you are actually going 65? My speedo reads about 2 MPH slow.
I have never looked at it real close but my Allison 3000 MH does not shift to the 6th gear until around 62 mph even with real modest acceleration. Then on flat ground at 65 MPH it runs at about 1500rpm. With a toad I get close to 9 MPG without the generator running.
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Old 11-07-2021, 07:38 AM   #13
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Originally Posted by Lowflyer1 View Post
Do you know for sure that you are actually going 65? My speedo reads about 2 MPH slow.
I have never looked at it real close but my Allison 3000 MH does not shift to the 6th gear until around 62 mph even with real modest acceleration. Then on flat ground at 65 MPH it runs at about 1500rpm. With a toad I get close to 9 MPG without the generator running.
I use my Garman for speed and my tach for rpm at 65 mph my tach is reading 1700 rpm. Really not sure if either or both are accurate. At 1500 Iím running about 60 mph. None of these numbers look excessive, but I donít have anything to compare them to.unless I have my gauges calibrated which I may have to do in order to know for sure.
I appreciate your input, gives me something to consider
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Old 11-07-2021, 10:37 AM   #14
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Quote:
Originally Posted by Tiffin0542 View Post
I use my Garman for speed and my tach for rpm at 65 mph my tach is reading 1700 rpm. Really not sure if either or both are accurate. At 1500 Iím running about 60 mph. None of these numbers look excessive, but I donít have anything to compare them to.unless I have my gauges calibrated which I may have to do in order to know for sure.
I appreciate your input, gives me something to consider
Not a scientific response, but my numbers are like yours regarding speed and rpm. Same engine and approximate weight. I can't see your gauges being so far off that your mpg would be as low as you describe.
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