5.9 vs 6.7 mileage

[HBarlow]

I've never calculated fuel mileage as poor as Sag2 reported but I have full confidence in the accuracy of his report. There is nothing at all unusual about under 7 mpg when you're running hard over mountains, pulling a trailer, and into or against a crossing wind.

 

[CUMMINZ]

My 06 dually never shined with fuel economy. This far into the new year I would wait on a 2012. Good things to come.

 

[Jim W]

In my humble opinion, there are several factors that affect fuel economy (mileage) on any truck. To me they are in order of importance.



1. Speed, this will uses more fuel as you increase your engine RPM's to achieve greater MPH.



2. Emission requirements on a new modern diesel engine. Burning extra fuel to eliminate soot will??? What can you say.



3. A larger displacement engine with more horse power and torque will use more fuel than a smaller displacement engine, 6. 7L Vs 5. 9L.



4. 4X4 Vs a 4X2 vehicle, a 4X4 will have more drag do to the running gear thus mileage will be down.



5. SRW Vs A DRW vehicle, A DRW vehicle will have more rolling resistance then a SRW vehicle.



5. A larger displacement engine with more horse power and torque will use more fuel than a smaller displacement engine. 6. 7L Vs 5. 9L



6. Mass, the more mass you move down the road then the more energy required to move that mass.



So with that being said I believe that all STOCK 6. 7L engines will never achieve the same fuel mileage as a STOCK 5. 9L engine. They will be within 2 or 3 MPG but will never be the same. The laws of physics will not let it happen.



If you want the best fuel mileage then slow down max speed of 55 MPH, buy a 3500HD, 4X2, regular cab short wheel base with a SRW truck. This should be the most fuel efficient truck that Dodge offers for you.



You either buy a new truck because you need a new truck or you want one. Do not base your decision on fuel efficiency over a 5. 9L Vs a 6. 7L engine because the 6. 7L will always lose.

Jim

 

[SOLER]

#3 (and #5 since you mentioned it twice) not necessarily true. The 8. 3L Cummins "C" is more fuel efficient than the 5. 9L "B" And just because it makes more horsepower doesn't necessarily mean it uses more fuel. If it can make that power more efficiently, it will use less fuel to do it. However, in the case of the 5. 9 vs 6. 7, this is true. The whole reason for the 6. 7 was to make the same horsepower as the 5. 9 while making emissions "numbers" what they "needed. " The whole premise of the 6. 7 was to make enough horsepower to make customers happy, while cow-towing to the EPA with all the emissions crap. And oh by the way, my 2010 6. 7 is putting more pollution and poisons in the air than my 2001 because it's burning more fuel, thus making more pollution, while at the same time using up fossil fuels quicker.



Just Sayin'



SOLER

 

[MChrist]

I, too, would respectfully disagree with the statement that larger engine size = lower fuel economy. At work, for example, our class 8 trucks are all prepared equally. Only difference is cab style and engine. Weights are the same, duties are the same, routes are the same, but our Freightliner with Cummins M-11 consistently delivers 6. 5-6. 9 mpg on it's routes versus our Cat C7. With the Cat, you are always on the throttle, compared to the Cummins, where you rarely are. We even switched drivers for 2 weeks thinking it was driver input, and numbers came back the same, ruling out driver input.
As far as normal sized vehicles go, I think people automatically believe smaller engines automatically give better mpg, but then again the smaller engine is also usually in a smaller vehicle. So in a way, I suppose you could then say that a vehicle with a smaller engine will provide better mpg than a bigger engine, but not necessarily in the same vehicle.
Any who... lower engine speed also doesn't automatically equal lower fuel consumption. All diesel engines have their "sweet spot" where they provide a higher level of power per volume of fuel, aka Brake Specific Fuel Consumption, (BSFC). Cummins 5. 9 is somewhere around 1900 I think. However, I get better fuel mileage at 1500. Why? Because (among other reasons) there is a larger differential between air drag vs fuel efficiency. My motor may be 10% more efficient (for example only) at 1900 vs 1500, but my aerodynamic drag may increase by 30%. So just going by the best BSFC number on a graph isn't enough. And the less aerodynamic you are, the larger percentage of drag you will incur, since drag doesn't increase equally with your speed, it ramps up substantially (on the "square").
But using my example, 1500 rpm vs 1900, wouldn't 1000 rpm be even better? Probably not, since now the engine is lugging, and calling for more fuel, and there may not be enough air/boost to efficiently burn the extra fuel that's getting called to the party.

So I agree with most of what you said, that being that mpg is a factor of efficiency, when all things are equal. Throw in EPA requirements, and it all goes out the window.
Hope I didn't make things more confusing...

 

[JHumphries]

Hope I didn't make things more confusing...

Clear as mud. :-laf

The sad part is, it makes sense to me.

 

[Powder Extreme]

MChrist is absolutely correct, just because an engine is bigger / has more hp doesn't mean it will get worse mileage.

Last year a friend and I took his little Suzuki to Las Vegas running 75 - 80 mph on the way down, he averaged 26 mpg. On the way home we were driving 55 - 60 mph (following one of my military vehicles) and he STILL only averaged 36mpg.

A few months later we did the exact same trip only this time we drove my Audi which has probably around 3 times the horsepower and is definitely heavier. On the way down running 80 -85mph we averaged 34mpg. On the way home running 55 - 60mph we averaged 46mpg!

His poor little Suzuki was running it's *** off to hold 80mph up and down all the hills while my Audi was simply loafing along at 85mph, that's where the fuel mileage went.

 

[jhenderson]

I have to disagree with the statement that rpms are the reason for fuel mileage. I believe it is aero drag and mass at speed. I can plow snow (12 inches on the flat) all day long, and run at approx 2000 rpm doing it, on 1. 5 gallons per hr. If rpms were all that mattered, at that rate, I should be getting over 40 mpg on the highway. I am luckey to break 18. Speed is the factor, no doubt, but it's drag, not rpms, that kills our mileage.

 

[1971]

Anyone know the RPM drop with 6 speed automatic vs. 4 speed automatic in my 5. 9 with the same 373 rear?

 

[RustyJC]

60 MPH in 6th gear (2011 dually tire sizes):



3. 73 gears = 1488 RPM



4. 10 gears = 1612 RPM



Rusty

 

[Jim W]

Gentlemen,

I still stick to my statement that a 5. 9L engine will get better fuel mileage than a 6. 7L engine.

An I quote again " A larger displacement engine with more horse power and torque will use more fuel than a smaller displacement engine, 6. 7L Vs 5. 9L. "



My state pertains to HP and Torque in order to achieve this the engine needs to burn my fuel to increase the HP over a 5. 9L engine.



As an example: Take a stock 3500 HD 5 speed manual transmission with 3:73 gears, 4X2, SRW, on a dyno. The same load and driving conditions are applied to both trucks. Put a stock 5. 9L engine in the truck with one gallon of fuel. This truck will go farther than a similar truck equipped the same way with a 6. 7L engine.

This is the only true way to measure accurate fuel mileage. All varaibles have been removed from the equation.

Jim

 

[rscurtis]

You are probably right, as the 5. 9 and 6. 7 are identical engines. MChrist was citing examples of two completely different engines. Here is another example. My '83 Mercedes at 65 MPH will do right around 30 MPG. That's with a 3200# car powered with a 3-litre diesel engine. My signature truck weighs twice as much, has twice the displacement, and almost 3X the horsepower, but still gets 24 MPG at the same speed, and that doesn't take their aerodynamic footprints into account. Some of the efficiency difference can be attributed to better gearing and a lockup TC, but the rest is due to the modern design of the Cummins compared to the old pre-chambered 2-valve engine in the Benz.



When I had my rental business, I had an International 4300 flatbed with the DT466 engine. It could do 12 MPG at 65 lightly loaded (empty wt. 14,700#), a figure that an F450 with a 6. 4 could only dream about, and this was way more truck than a 450.

 

[Tryingit]

MY 04. 5 5. 9L never got 20 mpg running around town. My communte to work netted me an average of 16-17mpg in the 04. My 2010 is getting14-16mpg. But I tend to go a bit faster in the 10. It is smoother and more refine so I don't realize it. The 04. 5 got 21 mpg on the highway on a trip to Florida doing the speed limit the entire time. That is with a Smarty JR set on eco. I certainly wouldn't expect this truck to get that. But I would think 18-19mpg would be realistic

 

[MChrist]

I don't have a 6. 7, but from what I have read and absorbed, 18-19 is realistic... providing you remove all emission related devices!! :-laf

 

[HBarlow]

Gentlemen,
I still stick to my statement that a 5. 9L engine will get better fuel mileage than a 6. 7L engine.
An I quote again " A larger displacement engine with more horse power and torque will use more fuel than a smaller displacement engine, 6. 7L Vs 5. 9L. "

My state pertains to HP and Torque in order to achieve this the engine needs to burn my fuel to increase the HP over a 5. 9L engine.

As an example: Take a stock 3500 HD 5 speed manual transmission with 3:73 gears, 4X2, SRW, on a dyno. The same load and driving conditions are applied to both trucks. Put a stock 5. 9L engine in the truck with one gallon of fuel. This truck will go farther than a similar truck equipped the same way with a 6. 7L engine.
This is the only true way to measure accurate fuel mileage. All varaibles have been removed from the equation.
Jim

Of course, there is no doubt that you are correct.

Folks who have the ability to manufacture their own facts may disagree but I will continue to agree with you on that.

 

[HBarlow]

My own '08 C&C surprised me over the weekend. My wife and I took a 300 mile trip down into Central Texas without a trailer. We were not in a hurry so I set the cruise control at 65 mph and relaxed. There was no noticeable wind.

The overhead monitor slowly crept up to 21. 4 mpg and stayed there until we left the highway and did some slow speed driving around town with a few starts and stops. It produced the same results on the way home on Sunday morning.

I think this is the highest indicated fuel mileage I've ever seen with this truck. It now has 97,000 miles on the odometer. I don't know what the hand calculated mpg was and really don't care. I have never hand calculated the mileage of this truck, even for one tankful. I accept it as it is what it is.

 

[RustyJC]

As I get more miles on the 2011 (a little over 2500 so far), mileage is improving. On a run from Athens, TX back to Cypress, TX yesterday with a mix of backroads and I-45 with the cruise set at 65 MPH, we averaged 10. 4 MPG towing our 16K RV (see signature). That's as good as we would get with our 2002 that averaged 10. 0 to 10. 5 MPG towing the 5th wheel with some excursions below 10. 0 MPG when running hard in hilly terrain or against strong headwinds.



On a previous trip from Cypress to San Antonio and back towing the 5th wheel with the 2011, we saw 9. 2 to 9. 5 MPG.



Rusty

 

[MChrist]

Rusty, those mpg #s are what I would expect towing a monster trailer. I am still shocked at Sag2's 6. 8 mpg though.
Harvey, when my '06 was bone stock and my OLM (overhead lie-o-meter) indicated 21-22, my hand calculated mpg was in the 14-16 range. Some people say their OLM is accurate, mine never was. With Smarty, it's off even more.
For reference, my '96 Tahoe's best highway mpg was 15. 8 at 55, all highway, no a/c, no noticeable wind. It has 3. 73 gears. At 65 and similar conditions, around 14.

 

[1971]

Is anyone running 315 70 17's with a 373 rear on the new trucks. Still trying to figure out which gear ratio to get.

 

[HBarlow]

Rusty, those mpg #s are what I would expect towing a monster trailer. I am still shocked at Sag2's 6. 8 mpg though.
Harvey, when my '06 was bone stock and my OLM (overhead lie-o-meter) indicated 21-22, my hand calculated mpg was in the 14-16 range. Some people say their OLM is accurate, mine never was. With Smarty, it's off even more.
For reference, my '96 Tahoe's best highway mpg was 15. 8 at 55, all highway, no a/c, no noticeable wind. It has 3. 73 gears. At 65 and similar conditions, around 14.

You could be correct, I can't prove or disprove what you suggested but I don't think my overhead indicator is as inaccurate as some are. It usually shows around 16-17 mpg when running fast on the interstates and usually around 10 or 11 when towing my trailer.