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Extremely Poor MPG Killing Me

I talked to Kevin on the air about this problem 2/27. I have a 2013 Pete 587, 2012 Cummins ISX15-2250 @500HP 13 speed with 3.70 rears. At 70 mph I am sitting at 1450 RPM. Ever since I bought this truck in January I just can not get it over 5 flat most of the time. I can be running 65, 70, 75. It doesn't seem to matter. Logic dictates I should be seeing roughly 6-6.5.

I run LTL so speed is important. Many of times I have to fly through TX/KS at 75 just to make an appointment. I slow down when I can but that is not always an option for me. Still I should be seeing better than 5?!

58 CPM fuel alone is killing me. Anyone have any ideas on what I can do without breaking the bank on mods?

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Hi Michael, I think I remember listening to the call too. Please correct me if I'm wrong but I thought I heard you say that the truck doesn't pull well, that its kind of a dog and falls on its face when pulling a hill. If this is true, it doesn't make much sense. A ISX 500hp with a 13spd and 3:70 rears should pull fantastic. I'm wondering if there is something weird going on in the ecm and if the engine is even producing the rated horse power. Maybe doing a dyno test first to confirm that the engine is producing the torque and horsepower it is supposed to would be a good idea.

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I heard the call, I agree mostly with Kevin except about the trailer to cab air gap. 30 inches or so. Aerodynamics are extremely critical at those speeds. If you are going 70 into a 30 mph wind, that is a 100 mph wind! The winds and temps this winter have been crazy. I have struggling to get 6mpg at 61 mph in South Dakota, Minnesota, Nebraska, and Wyoming. Michelin X-1 singles tractor and trailer, will probably save the most fuel with one modification. The Scanguage KR may save almost as much for $200. Air tabs will probably not give a measurable gain in my experience. The guys in the Facebook groups "Mpg Masters" and "9 mpg club" have fantastic, well monitored pages and great advice. You probably have 3:36 rears. My truck with those turns 1400@ 65. Even if you go fast, accelerate slow and smooth. No cruise control except on VERY flat roads. Keep it tuned up! NO idling.

Good luck!

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Re-reading the question, 2250 ft/lb torque is a lot! Start with the Scanguage KR to learn to handle that monster!

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Can't do much bout the gap with the extended frame. I have about 5 feet between cap and tractor. I do have air tabs on this thing. I want to get the X-Line Energy as they have very low rolling resistance. Also thinking about flow below and Fleet Air Filters. My wife has a good job and hold the fort down while I'm out here so I can recycle a lot of my cash into fuel saving features to lower my operating costs quicker.

Here is what I am looking at in the near term:

In purchase order and estimating fuel saving:

1:Fleet Air Filters .2

2:Flow Below .2

3:Pittsburgh Power Box .3

4:Pittsburgh Manifold.2

5:X-Line Energy - Full Set .4

6:Pittsburgh Mufler .1

7: FASS .1

That a nice starting point for the next 2 quarters I think. By my estimation off median manufacturers specs that alone should get me around 1.5 more. With 14k in th box and avg speed of 67 from Toledo to Rochelle IL I am seeing 7 right now. Did a hook and book at the yard in Batavia for 38k in the box and that number has dropped to 6.9. Going to run 65 across IA down to KC. figure I may get 6.5-6.7 out of this tank.

On a side note, I do not idle. I have a TriPac and rarely even run it. Can use the bunk heater without the Apu and I keep it set low at 55ish. I only star the apu to charge the batteries.

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Check into shortening the frame also, $1500-$2000, may have a .4 mpg improvement for quick payback. I would put the scan gauge KR at the top of the list. 2-4 week payback!

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On a test today over 200 miles between Des Moines and Lenexa KS I got 5 flat at 65. I was fighting some head wind so maybe that had a hand in it. Through research I have found that I have a .73 OD in top gear and 12th is my direct. So single OD tranny. By my research with 24.5 tires, 1300 rpm - just above my peak torque at 1200 @ 1850 - is 65. That should put me in the peak fuel mileage range?? I'm getting the same economy at 65 that I was getting at 70-73. Sometimes it feels like it has power going up a grade and sometimes it just falls flat on it's face. There has to be a bad injector I think. Will know more when I send Kevin that oil sample in a week or so. Will probably do my oil change sooner just to get it to him sooner. This is driving me crazy!

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Correction:

After further testing using known factors - in the case 3.70 rears - I have pinpointed exactly what I am working with.

I have a 13 double over .73 and .87 with direct being 11th. Obviously..

By my calculations currently I should have a road speed of 65 in top gear aiming for a target of 1400 RPM. For the ISX engines, Cummins recommends 1300-1350 for optimum fuel performance with 1400 being the max for the higher HP engines.

I am all but certain the rears are playing a large role in reducing my economy. Here is part 1 of my potential solution.

Option 1:

Swapping to 3.08's would put me at 66 mph in 12th leaving me room to go up to top at 73 @1400.

Option 2:

Swap to 2.64 would put me at 67 @1400 in direct. That right where my engine is most eco. These cummins you can run all the way down to 1100 with peak torque of 1850@1200. Direct I would need to downshift at 53. Having the 2 OD gears would give me the option of 66 @1200 on the flats in 12th and 13th at 73 roughly around 1300 if needed.

Note: I like the idea of running in direct mostly to minimize power loss and maximize eco. Having the double over available for those times I have to go is good but I would rarely use 13 unless I'm in west TX.

Option 3:

Swap 2.85's. Direct will give me 62 @ 1400, 12th 66 @1300. and 13th 73 at 1200.

Note: I do not like the idea of my direct being 62 at the top of my optimal torque range. I do like having my OD at good cruising speeds for when I need it at lower RPM.

So, what do yall think? I'm more looking towards option 2 or 3. I know going faster is going to blow my fuel mileage to !&&* to I will avoid it as much as possible. Just sometimes I am going to have to go go go! On a side note, ordered the scan gauge today.

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Hi Rick. You are indeed correct that's why I am so confused about this. Watching my Pete gauges on the infotainment system the ECM is even showing that full throttle up a hill I am only producing 90% of the rated torque. So 1850*.9 = 1665. That's a lot of pulling power wasted. That is one of the reasons I brought up possible injector failure. My last truck had the same engine with a 10 speed OD and 3.55's and it would run away from this dog.

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Oh, I forgot to mention. Coming up 35 north out of KC at the 435 split, this thing flat fell over itself, had to go down to direct. Best part was - I WAS EMPTY!!!!!!!!!!!!!!!!!!!!!!!

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Michael will be eternally grateful.
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