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Old 11-14-2016, 09:52 PM   #1 (permalink)
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Cummins 6.9L turbo diesel economy

Hello, I have a question for you, after reading the hypermiling section I have done very well increasing MPG of my 2014 Ram 2500 from the low 13s to the mid 15s. I've been trying to get more efficiency out of it (what seems reasonable given my typical driving situations), seen all sorts of suggestions for accelerating, and I'm not sure which make sense for my engine. It sounds like quicker acceleration is better than slow, which makes sense to me and playing around with it I see a definite difference going harder on the pedal than grampa, but I'm sure there's a point at which this becomes as bad as or worse than creeping up to speed. Like flooring the pedal to the speed limit then coasting (I got here searching for pulse & glide). The consensus seems to be that my engine sips fuel best at 1600+/- RPM (peak torque according to Cummins), and for me that's between 60-65 on the freeway. So would accelerating trying to keep the tach as close to 1600 while it gears up make sense? I drove a friend's Ram with a driving coach tuner and as I recall it showed where you should be as you accelerate in a red-yellow-green band sort of display, unfortunately I wasn't paying attention to the RPM.

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Old 11-15-2016, 02:33 AM   #2 (permalink)
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I'm guessing you have such a low RPM at 65 due to large tires? If so, they are probably hurting your MPG. 15 MPG in a Cummins isn't great.

Focusing on acceleration won't give you as much improvement as focusing on rolling resistance, aerodynamics, and avoidance of using the brakes.

I put tall and wide tires on my 5.9L and the MPGs dropped. The front end is not good, especially since the track bar wears out almost instantly. I've had issues with calipers dragging too. On top of all that, my tranny is going. I'm still able to get 20 MPG at 65 on the freeway.
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Old 11-15-2016, 12:10 PM   #3 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
15 MPG in a Cummins isn't great.
I agree, even my gas Sierra could flirt with 15MPG if I was going ~60MPH.

Do you have any mods done to the truck? Oversized or offroad tires? Lift? Tune?
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Old 11-16-2016, 12:43 AM   #4 (permalink)
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It's called DPF and regen. That RPM is better then the older ones as that truck has a better transmission. To bad they didn't have the newer 6 speeds with the older, pre-emission 5.9's. That would have been killer.
Anyway, by keeping the EGT's low you improve economy, but then you get an active regen cycle more often which dumps raw fuel to burn off the DPF. Driving it a little harder may keep the DPF clean without regen. What is best for economy I have no idea. Delete the DPF and deactivate regen would definitely help.

I see a youtube video that claims 1112 degrees is where passive regen occurs. It also says if the active regen cycle is started and you turn off the motor before it finishes, it will start all over on the next startup. It is probably not 100% accurate for your application but an interesting video.

Last edited by Hersbird; 11-16-2016 at 01:01 AM..
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Old 11-20-2016, 04:36 PM   #5 (permalink)
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Some of the rules that apply to gas engines don't apply as well to diesels. Gas engines are most efficient at high throttle (accelerating briskly) in part because closing the throttle plate and generating vacuum robs power and efficiency from the engine. While some do, many diesel engines don't have throttle plates, and a better rule to follow is just to keep RPM down.

You might want to check this thread out:

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Old 11-21-2016, 06:54 AM   #6 (permalink)
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For me and my 1st gen 91.5 I keep the egt about 700-750 to speed then about 500 or less in town and 800 to 65-70mph dr p to 500 flats 600 slight incline 700 about 5%grade 800on 7% with an occasional bump to 900 to accelerate and maintain OD (shift point 59 mph@above 825egt)
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235/85R16 Revolutions per mile (km)635.65 (394.97)
3.55gear , .69OD =1557rpm @60mph +5% (for no lockup) = 1634 rpm .peek tork 400ft lb @1600
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Old 11-21-2016, 12:23 PM   #7 (permalink)
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I have taken a 2012 3500 flatbed with 6.9l geared for towing( i think 3.9 or 4.10 rear end) for an 800 mile trip, and it would hold just under 19 mpg unloaded until i hit a regen, it droped to 18 when it was done. After that i would squeek it back up to 18.5, then back to high 17 on next regen. All said and done i got about 17.5 mpg for that trip.

The regen cycles and towing gears really do kinda kill getting good mpg.

A mid 2000s 2500 with 5.9 I held to 25 to 26 mpg for a 100 mile trip parking lot to parking lot but i dont think that was geared for towing as it held low rpm while cruising at 60 to 65. NO regens on those older trucks.
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Old 12-01-2016, 02:45 PM   #8 (permalink)
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Quote:
Originally Posted by gumby79 View Post
For me and my 1st gen 91.5 I keep the egt about 700-750 to speed then about 500 or less in town and 800 to 65-70mph dr p to 500 flats 600 slight incline 700 about 5%grade 800on 7% with an occasional bump to 900 to accelerate and maintain OD (shift point 59 mph@above 825egt)
no tack so heargos
235/85R16 Revolutions per mile (km)635.65 (394.97)
3.55gear , .69OD =1557rpm @60mph +5% (for no lockup) = 1634 rpm .peek tork 400ft lb @1600
18.xxx avg naked 24.xxxmpg with cap
Beautiful, so I can believe what Cummins has been telling folks (with no actual data or even an explanation). And not to offend anyone but I keep hearing about claims of higher MPG than the factory MPG with no mods or fooling around with the EGR emissions etc. and that might be what a hypermiler gets but you can only piss off so many people on our local roads. And these engines are supposed to be getting less efficient in each gen as the EPA cracks down on MPG. I bought it precisely because it was the last model that didn't require the ridiculous DEF. I'm more curious if anything but saving on guzzle even if it's only $10 a tank adds up. And for the record my truck is 100% stock and the tires are actually an inch smaller because I need an aggressive tread and the factory size tires are $400 more for a set. Also I have a steel canopy that it sure adds weight compared to fiberglass. Right off the dealer floor (almost NOS, I think the guy who sold it back to them kept it in his driveway for the looks) I got 15.7 on a couple hundred mile freeway trip @ 65 on cruise control not even knowing how to drive economically yet. Haven't seen that since so regen could certainly be a factor. I don't do enough freeway miles for it to kick in at a more optimal speed (or so I have been reading). Thanks everybody for your input!
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Old 12-01-2016, 02:57 PM   #9 (permalink)
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There's no such thing as "factory MPG". I assume you're referring to what EPA testing suggests the typical driver will get?

Hypermiling doesn't have to piss people off, either; it depends on what you're doing. Certainly tailgaiting large trucks and driving 25mph under the speed limit can be annoying, but there are plenty of other safer ways to save fuel. Remember that every time you push the gas pedal down you're burning fuel to gain momentum, and every time you press the brake pedal you're turning forward momentum into brake dust.

An example of a driving habit that can save a ton of fuel but not piss people off is situational awareness. For example if the light ahead of you turns red, don't keep on the gas until the last minute and then slam on the brakes, but instead take your foot off the pedal and take your time getting there. Nobody is leaving the redlight any sooner than anyone else, and by doing this alone you can often beat the EPA city rating by 25-50%.

Weight is far less important than aerodynamics on the highway. Any roof racks or bits you bolt on to the outside are basically small parachutes catching the wind and pulling your truck to slow down.
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Old 12-01-2016, 04:02 PM   #10 (permalink)
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Your truck may not have the urine injection but it definitely has a DPF and regeneration modes. You need to learn when those are happening and how to limit them, and never interrupt them (or they just immediately start all over again from scratch). That or just get rid of it. The 6.7 can get good mpg even bone stock. TFLtruck has a YouTube video of one doing a empty loop where they get over 20mpg. Then again their Ecodiesel Ram did 33mpg on that loop which is why that's the truck I really want.

Ps, I have a friend with an 2012 and he is only getting 15 but he is going 75. At least he says pulling a horse trailer it only drops to 13. The canopy actually probably hurts aero compared to an open bed or flat cover, steel or fiberglass. The smaller tires also hurt especially with aggressive tread. They increase highway rpm and I bet they are just as wide giving no reduction in frontal aera.


Last edited by Hersbird; 12-01-2016 at 04:12 PM..
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