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Old 08-31-2011, 07:42 PM   #127
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Purman, actually it is, the other posts are off topic as this one is. jim
Jim,
Who died and left you in charge....your not, so get over it.... if someone posts a bit off key..thats life, esp if an apology is offered..No one really needs a hall monitor at this age do we??
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Old 09-02-2011, 08:44 AM   #128
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More Fuel Mileage Information

The new truck now has 2400 miles on it. A couple of days ago, we pulled Lucy on a short trip from Jacksonville, Florida to Hardeeville, South Carolina (160 miles) at 70 mph. Fred's fuel mileage was a little less impressive. He averaged 13.1 mpg. We are heading back to Jacksonville today, and we'll see how he does.

Brian
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Old 09-02-2011, 08:59 AM   #129
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The new truck now has 2400 miles on it. A couple of days ago, we pulled Lucy on a short trip from Jacksonville, Florida to Hardeeville, South Carolina (160 miles) at 70 mph. Fred's fuel mileage was a little less impressive. He averaged 13.1 mpg. We are heading back to Jacksonville today, and we'll see how he does.

Brian
Brian, diesels are less sensitive to load than gassers, but are sensitive to RPM. Diesels make peak power at 1800 RPM, peak efficiency on either side of this. When driving the diesel, try to set your speed based on RPM not mph, This will take some experimentation on your part. You have the 6 spd Alli, same as us, some speeds I reach peak efficiency; 59 mph, 63 mph, 67 mph, 69 mph, 73 mph. Depending on where we are, what the load is, etc…I pick one of these speeds and lock the cruise control. All of these speeds leave us right where we should be in the RPM range. I don’t tow any faster than 69 mph, unladed I will drive 73 mph on the freeway if conditions allow…and surprisingly get very good mileage at this speed.
The other variable of course is friction-your “profile” will help determine some of this (kayaks on the roof, bikes, etc).
So try some different RPM (“speeds”) and take some notes.
Keep in mind, your truck is brand new and mileage will improve. I saw some improvements ~15k miles, 25k miles, and recently ~50k miles. We now get 16 mpg towing the AS.
Have fun,
Bill
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Old 09-02-2011, 12:23 PM   #130
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MPG drops like a rock above 60-mph. Brand, motor, etc are irrelevant. There are no ways to fix this via gearing, etc. The best mpg will always be at or below this speed for highway travel. Second, the vehicle is designed to provide best towing performance between 55 & 62 mph in all aspects (handling, braking, acceleration and fuel economy), again, regardless of brand. I find that a tow speed of 58-mph gives me room to accelerate for the hill ascent, and to drop off in speed some to get a heavy vehicle around me when passing. And very little throttle movement, a key to steady-state highway mpg, when doing either of these.

That D-Max ought, like any other vehicle nowadays, see incremental improvements in FE through 30k miles. For highway travel, though, the cruise control set speed that gives the lowest rise in manifold pressure and exhaust gas temp while maintaining speed up and down unremarkable rises will also be the one that gives best mpg. The sweet spot.

For someone serious about mpg then a search for a BSFC map of the engine will give fuel burn rates versus rpm that allow for the most intelligent choices.

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Old 09-02-2011, 02:21 PM   #131
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The mods are selective from what they consider offtopic---really is a problem on this forum from my experience. So yes I am in charge of my activity here! Good bye. jim
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Old 09-02-2011, 05:43 PM   #132
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First trip milage

Just returned from my first long trip pulling the AS, 2000 miles. Avg 13.8 towing fill up to fill up, 14+ when mixed with non towing mileage. When I kept it at 63 or lower I hit the 1/4 tank increments like clock work at 100 mile intervals less if above more if below. One long stretch of constuction at 50 avg pushed 14.2 mpg. and 130 miles at 1/4 tank increment. Current mileage on the truck is 48000. I got 11-12 mpg pulling horse trailer from new to 25000 miles. Not sure what that is attributable too... engine break in vs configuration of load.

I met up with another AS identical size pulled by SUV V8- he reported 9 mpg. I calculated using .30 difference in cost per gallon a $240 savings for the trip vs gas. I am happy!

I did not monitor RPM but I will next time. But I do agree that anything above 60 mph decreases fuel economy exponentially when towing.

Anyone use fuel additives? I was told that diesels before '07 needed additives to combat the effects of low sulfur fuel.
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Old 09-02-2011, 07:22 PM   #133
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I made the same trip back to Jacksonville today. I tried keeping it at 65 mph to see if the fuel mileage improved. Today's trip netted a 12.9 mpg average. That's about a 7% mpg reduction than the 70 mph run. I'll start experimenting for Fred's sweet spot.

Thanx for all the tips, guys.

Brian
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Old 09-02-2011, 08:32 PM   #134
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Here's the sweet spot for my '06 Duramax. I like to tow at about 62-63 mph, conditions permitting, which puts me right about 1700 rpm. Picture taken during a 2200 mile trip this spring as previously described earlier in the thread, towing our 23' Flying Cloud. I agree with BillTex, it varies a little, but this is in the max torque/power band area. With the Alli 6 speed, the Duramax just loafs along; no stress, no strain.
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Old 09-02-2011, 09:57 PM   #135
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nice looking rig!
x2!
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Old 09-02-2011, 10:02 PM   #136
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I made the same trip back to Jacksonville today. I tried keeping it at 65 mph to see if the fuel mileage improved. Today's trip netted a 12.9 mpg average. That's about a 7% mpg reduction than the 70 mph run. I'll start experimenting for Fred's sweet spot.

Thanx for all the tips, guys.

Brian
Brian,

You may have had a DEF regen during this trip, and that will hit your mileage.
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Old 09-02-2011, 11:47 PM   #137
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Brian,

You may have had a DEF regen during this trip, and that will hit your mileage.
I have read some about this DEF regen on the Diesel Place Forums, but I don't know if I understand what it is. Could you please explain further?

Thanx, Brian
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Old 09-03-2011, 09:08 AM   #138
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Hi all

I have been pulling a 30 footer with an 2006 DuraMax for 4
years now, and here are my thoughts:

The slightest amount of head or tail wind can make a big
difference in mpg performance. I range between 11 and 16
on flat roads at 60mph. With no wind, I average about 14.5.

Although there seems to be no proof that driving near
your engine's peak torque generates best performance,
from experience I think it does (at least with a diesel engine).
Several truck drivers I have talked to concur. 60mph in
6th gear gets me at peak torque 1600rpm.

Finally, and a little off topic, you got to love that
Allison transmission in tow mode. I hardly ever use the
brakes. Actually, I have towed over the continental divide
on I70 without touching my brakes, amazing.
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Old 09-03-2011, 11:37 AM   #139
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I have read some about this DEF regen on the Diesel Place Forums, but I don't know if I understand what it is. Could you please explain further?

Thanx, Brian
Don't confuse DEF with DPF.

DPF (diesel particulate filter) is what costs fuel mileage. It is a soot filter, which when gets full), will command extra fuel to be injected late so it burns in the exhaust. This burns the soot into ash which remains in the filter, albeit at a much reduced volume. This cycle is a function of fuel consumed and fuel cleanliness. Approx. every 100,000 miles or so, the filter will need to be removed and the ash dumped out.

DEF (diesel exhaust fluid) has no fuel mileage penalty. It is a fluid (urea based) which is injected into the exhaust stream. It acts as a catalyst to convert NOx into nitrogen and water(?).... I think. Anyway, it takes a pollutant and converts it into inert gasses...
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Old 09-03-2011, 12:22 PM   #140
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I have a 2005 silverado 3500 with the Duramax and Allison 6 speed transmission and have been getting over 50 mpg while towing. As others have said the secret is RPM. By adding the Eaton 2 speed differential and using a blend of jet fuel, water and acetone it is not uncommon to get these superior results. Additionally, water vapor creates absolutely beautiful contrails behind the airstream. My next mod is to add a duel exhaust system in an attempt to create a dual contrail. My only problem has been towing at speeds over 100mph I get an overtemp warning from my tire pressure monitoring system and stopping distances have increased substantially.

Disclaimer: All data was collected with a vehicle equipped new tires, new brakes, a recent wax job and production test pilot at the controls. Your actual results may vary
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