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Old 06-19-2013, 09:30 AM   #1
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5'th at 60 or 6'th at 70..best fuel mileage?

I am getting our Cummins ISB/Allison 21XX 6-speed combination figured out from a "when does it shift" perspective, but am now realizing the tremendous power consumption of the cooling fan (this is a front engine diesel). My mission is to increase mileage and also to figure out how to keep the engine temp down thus causing less cooling fan time (quitter and drawing less power).

So I see two options...I can cruise at about 70 miles an hour to keep it in 6'th (usually fast enough that it stays in 6'th even if the cooling fan comes on). This takes a fair amount of pedal to the floor to keep it in 6'th when driving in hills, or I can drive about 58-65 in 5'th (cooling fan comes on less often). This also does better in the hills because it can hold 5'th all the way down to slower speeds.

Driving slower and getting there later is not a concern, but increasing fuel mileage, less noise from the cooling fan, and causing less strain on the engine and transmission is my interest. I am refilling the fuel tank frequently to calculate mileage and trying the two methods to see if driving slower in 5'th provides better mileage (two fills on driving faster in 6'th both came out at 7.1 MPG. When we leave our current camp spot I will try driving slower in 5'th and will calculate mileage.

Does anyone else have experience calculating mileage between driving faster in 6'th or slower in 5'th?
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Old 06-19-2013, 10:47 AM   #2
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The wind resistance of pushing the barn door , that is the front end of a class A, down the road at 70 would soon put your mileage in the dumpster.
Your shift points are curious to me though. I know I'm comparing a FRED to a DP here, but my 5th,6th shift and convertor lock up is 57mph, and soft cruise set at 60,my speed that gives best mileage, will hold 6th even during the hydraulic rad fan cycle, a 35hp. hit. Charts list an additional 50 hp. required to move a coach at 70 vs. 60 mph.
JMHO. Check your RPM at 60 mph , 5th and 6th if possible , if your trans won't shift into 6th at 60 , I'd be asking Allison about a program change, that will allow that shift.
Manual downshift on hills to keep the engine in the 2000, rpm range for power & cooling, is the way to handle the hills.
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Old 06-19-2013, 11:21 AM   #3
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Great information, thanks! I will record speed and tach settings...good idea
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Old 06-20-2013, 06:07 PM   #4
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I agree with the wind resistance comment at 70 mph, pushing a barn door.

You will be surprised that your mileage will be better at 55 in 5th vs 65 in 6th gear.

PS Mine also (3000 MH Allison) shifts into 6th gear at about 56 mph.
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Old 06-20-2013, 09:26 PM   #5
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I have the Cummins ISB 340 HP and I can force it into 6th at just over 55 mph, then hit the resume on the cruise for 55 and it will stay there until hill climbs. On the down hill it will shift back into 6th and hold on the flats. Mike Cody at Freightliner said it is not supposed to work that way on the Allison 2500, but it does.

I have also cruised in 5th at 55 mph, and even 60, when I forgot to hit the mode switch and saw no difference in MPG.

According to a recent article I read, for a car gas engine, a 30% increase in fuel economy can be seen at 55 mph versus 65 mph.
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Old 06-21-2013, 07:42 PM   #6
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In my 3126e 330 kitty pushing a 3852 Dutchstar I will get about 7.8 to 8.3 at 60 to 63 mph. But if a run 68 to 72 it drops to 6mpg at best. I like running about 62 and can drive it with a couple of fingers and is more comfortable than my daily driver. I have to go about 56 to 57 to get the allison 3000 to shift to 6th but then it can decrease in speed and hold 6th unless i come upon a grade. My daily driver is a 07 mbenz bluetec e320 that I drive 1100 miles a week. At 60 to 63 mph I will get 35 mpg but at 70 to 75 I will get 30 to 31 mpg . I also get better mileage when air temp is 70 or less in both vehicles. It just shows that pushing air faster is expensive. Just my .02. Safe Travels!
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Old 06-28-2013, 12:13 AM   #7
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Quote:
Originally Posted by Mystery View Post
In my 3126e 330 kitty pushing a 3852 Dutchstar I will get about 7.8 to 8.3 at 60 to 63 mph. But if a run 68 to 72 it drops to 6mpg at best.
Mystery, I've got a similar setup, perhaps a bit heavier, [3126/DS4090] and get slightly lower numbers, around 7.3-7.6 towing a 3000#toad. I drive at 1600 rpm, and set the cruise. That gives me about 63 on the flat, and I let Mr Allison worry about the shift points in minor hill country. In mountain areas we fall back to about 35-40 turning about 2000. Pushing 34000lb, that's about the best we can do.
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Old 06-28-2013, 04:57 AM   #8
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JFXG, your mileage sounds close to mine bc I do not tow yet. My Silverleaf show an average of 7.4 mpg over last 72,000 miles. I purchased this rig last summer with about 60k on it. My average is climbing slowly but I imagine the previous owner had a toad. It seems very close to yours if you factor in the toad and I typically weigh 27 to 28k. Safe travels!
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