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Duramax 6600 Welcome to the Internet's first Duramax 6600 diesel discussion forum for the LB7, LLY, LBZ, LMM, LML, LGH & L5P RPO code engines. Tips on performance, fuel economy, troubleshooting and more.

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  #1  
Old 05-17-2006, 11:32 AM
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Stlheadake Stlheadake is offline
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Default Maximizing LLY fuel mileage

I am new to this forum, and I have to admit new to Diesels.

My father had had his '93 6.2 since 95. It has been and continues to be a great truck. He is consistently getting 22-24 MPG HWY running 70MPH.

I have an '06 crew cab 4x4 with only a thousand miles on it. I have the LLY motor (darn it!), and it is bone stock. The only change or addition is a Line-X spray in Bedliner. I have stock wheels and Tires, no bug deflector or ANY changes to the truck. So far, I haven't gotten better than 16 unloaded on the interstate at 65MPH. I think I am running the 3.73 gears in the rear, but I don't know how to tell for sure (can anyone tell me how to tell?).

I am sure that as the motor breaks in that I can expect it to get a 'little' better. What is 'average' for most LLY users? I have read several threads, and mine seems low.

I bought the truck for the power, and pulling. But, the MPGs attracted me too. Now I'm not really seeing them. Sorry for being so long winded, but this brings me to my real question.

Would the addition of an exhaust system and possibly a cold air intake increase my mileage? The added horsepower would be good, but I am most concerned about mileage. Also, I pull my 6K travel trailer a couple of times a month. So towing mileage is sort of a consideration too.

I am sort of a MPG nut, and would like to see a little better than 16. My 1/2 ton Chevy with 5.3 gasser got 18 MPG REGULARLY at 70-75 on the interstate (Really!)
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'06 Silverado K2500 4x4 Crew Cab D/A "Big Max" AmpResearch retractable Running Boards, 4" Turbo back Kennedy Exhaust, Kennedy Custom Tune!
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  #2  
Old 05-17-2006, 12:22 PM
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More Power More Power is offline
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Hi Scott,

You've asked an important question, regarding fuel economy, and your truck's apparent lack of it.

First, all Duramax equipped 2500HD/3500 series trucks come with 3.73 differential gearing. The new 6-speed Allison with its double-overdrive will produce engine rpms roughly equivalent to a 5-speed Allison equipped truck with 3.42 gears. I assume you're running in 6th gear while on the Interstate.

The thread we began a few weeks ago describing a Scatter Plot would be a good place to begin when troubleshooting a low mpg truck. This requires an EGT gauge, and it might be worth installing as a first step (along with a boost pressure gauge). (Read the whole scatter plot thread - to get most out of it.)

We have a pretty good understanding of the relationship between EGT, boost pressure and fuel economy. It requires burning a certain amount of fuel to achieve a given speed. Exceptionally low mpg's must mean either your truck is producing higher than normal drag or the engine/drivetrain isn't running efficiently. Again, an EGT/Boost gauge would show that.

For example, a low boost pressure (due to some turbo or turbo control issue) would cause higher than normal EGT's and allow the truck to produce more smoke. You'll see this immediately with gauges.

IMO - The 2006 LBZ ought to be 20 or better when driven unloaded at 65-mph.

Jim
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  #3  
Old 05-17-2006, 12:22 PM
StrangeEngine StrangeEngine is offline
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Default

Quote:
Originally Posted by Stlheadake
I have an '06 crew cab 4x4 with only a thousand miles on it. I have the LLY motor (darn it!), and it is bone stock. The only change or addition is a Line-X spray in Bedliner. I have stock wheels and Tires, no bug deflector or ANY changes to the truck. So far, I haven't gotten better than 16 unloaded on the interstate at 65MPH.
In spite of a billion posts to the contrary, that's exactly the mileage I see with my '02 LB7 truck and have since Day 1.

I got ONE tank at 20.02 but I had the cruise locked at 55mph as a test. It ain't worth it.

I just don't see how other people are getting 18-20mpg at 70mph in the same truck as I have.

Quote:
I think I am running the 3.73 gears in the rear, but I don't know how to tell for sure (can anyone tell me how to tell?).
You've got the 3.73 gears because that's all the diesel truck is available with.

- Mark
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  #4  
Old 05-17-2006, 12:28 PM
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rob from bc canada rob from bc canada is offline
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Is it a Dually? (I think most use DRW in their sig for that), and is it a 3500?

These add wind resistance and weight.

Also, do you not have the higher ratio 6th speed overdrive tranny? If so you should be getting better than my 05

Is your mileage calculated from ODO readings and tank fills, or is that what the DIC says? My DIC is always 1-2 mpg optimistic.

In any case that mileage seems quite low, unless you're climbing a lot of hills at 65. I find with the weight of these things, they really suck up the fuel on hills.

Driving empty with mine (also bone stock) I get calculated averages of 17.5 US mpg driving mostly highway at 60-65 (with quite a few traficc lights), some local driving, and some medium hills mixed in. I record every fillup I put in it.

This has not changed for better or for worse that i can see since new, I now have about 8000 miles on it.

On straight flat highway at 65 I can get around 18.5-19 US MPG
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  #5  
Old 05-17-2006, 02:36 PM
WILLYD-MAX WILLYD-MAX is offline
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Don't expect to much better until you get more like 5k to 10 k on the odo. These duramaxes take a while to break in depending on your use. I drove about 2.5 tankfulls here a while back strictly highway and I averaged 18.8. And that was hand calculated with adding 2.5% for my 265s. BUT I tried to keep it at or under 2k rpms. I don't think people are going to see a drastic differance in the 6 speed ali fuel mileage at lower speeds... rpms. compared to the 5 speed ali. I also think using a good fuel injector cleaner as in FPPF Total Power on every tank helps mileage approx. 1 mpg. Just my experience with my 05....
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Dad's truck also (My daily driver)
2007 Classic Crew Cab 3500 Dually
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  #6  
Old 05-17-2006, 02:52 PM
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Stlheadake Stlheadake is offline
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Default milage issues

Thanks for all the replies. I WILL jump in and read ALL on the scatter plot thread. I DO have the 6spd Alli, and I DO run in overdrive. The truck is a 3/4 ton not a dually.

I just bought a case of stanadyne blue label cleaner, demulsifyer, lube, cetone, etc... I wonder what difference that will make. I haven't used it yet, I am waiting on the O'Reilly cetan booster and lube I used to run through.

I DON'T have a heavy foot, I run 70 mph unloaded and I take my time getting up there. So my mileage seems quite low to me. In all fairness though, I have not hand calculated what I am getting. I am relying solely on the DIC. This may be faulty.

So back to my ORIGINAL question: Will adding an exhaust and intake kit improve milage? I have read where some guys have even put in a performance chip and gotten as much as 5mpg (they claim) better than stock. Is this really true?

It seems that modification would pay for itself in about a years time!

I wanted thoughts from the experts before I plunk down the coin for the toys.
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St. Louis, MO


'06 Silverado K2500 4x4 Crew Cab D/A "Big Max" AmpResearch retractable Running Boards, 4" Turbo back Kennedy Exhaust, Kennedy Custom Tune!
'99 BMW K1200LT "Special K" my other "SUV"
'07 Salem by Forest River Fifth Wheel Toy Hauler
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  #7  
Old 05-17-2006, 02:59 PM
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Lightbulb

By all means, hand calculate the mileage. Try to use the same pump, and use the pump the same way each fillup. I normally just set the pump on the middle speed setting, and remove the nozzle once it kicks off.

A larger exhaust system and free-flowing muffler can have a small effect on fuel economy - but not the 3-4 mpg you're looking for. I'd get gauges first....

Jim
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  #8  
Old 05-17-2006, 06:15 PM
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Stlheadake Stlheadake is offline
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Default Following your suggestions

Quote:
Originally Posted by More Power
By all means, hand calculate the mileage. Try to use the same pump, and use the pump the same way each fillup. I normally just set the pump on the middle speed setting, and remove the nozzle once it kicks off.

A larger exhaust system and free-flowing muffler can have a small effect on fuel economy - but not the 3-4 mpg you're looking for. I'd get gauges first....

Jim
Thanks Jim, you and everyone else on this page are a true asset. I am going to order the guage kit from Kennedy first I guess. Othere than the pyrometer and boost, is there something IN a gauge that I should be looking for?
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St. Louis, MO


'06 Silverado K2500 4x4 Crew Cab D/A "Big Max" AmpResearch retractable Running Boards, 4" Turbo back Kennedy Exhaust, Kennedy Custom Tune!
'99 BMW K1200LT "Special K" my other "SUV"
'07 Salem by Forest River Fifth Wheel Toy Hauler
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  #9  
Old 05-17-2006, 10:10 PM
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More Power More Power is offline
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Gauge brand, model and mounting location are largely personal choices.

Have a look at the 2005 Gauge Update to get an overview of what's available for conventional gauges and what sort of mounting appeals to you.

Personally, I like the dual gauge pillar pod for boost/EGT, mainly for visibility - and they look cool there. The Edge Attitude comes in a close second, though I don't know about 2006 model year requirements. The vendors will know....

Jim
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Old 05-18-2006, 06:15 AM
David Proske David Proske is offline
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My 2003 and now my 2006 seem to be pretty close in terms of mpg. From what i've seen driving diesels for a number of years.... anyone claiming 20's at 70-75 is flat out fibbing. Or they are going downhill with a tailwind.
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