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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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Old 05-18-2006, 07:25 AM
beeler beeler is offline
Join Date: Aug 2002
Location: Fort Benton, MT
Posts: 123

I have to agree with David. The only way I could get 20 mpg out of my '02 is if I drive like I'm following old folks to church. On long trips on flat ground with no wind and cruise on 75 I get about 19. Most of the time 17-18. Less in the wind or if I'm in a hurry.

I have the Edge box and a 4" exhaust. I can't say that they really made much difference in economy. Maybe a little. Towing milage is better than stock if you pull the hills at the speed you would have stock.
2002 Chev D/A CC LB - it's gone
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Old 05-24-2006, 01:14 PM
Gizmo Gizmo is offline
Join Date: Apr 2002
Location: northern IL
Posts: 5
Default Improving Fuel Mileage

The cost of fuel has modified my driving a bit. Last weekend a 700 mile trip was done mostly with cruise control and 65 mph (slower than I usually drive). The tonneau cover was in place as a couple of threads suggest this does better than my tailgate down theory. A calculated 21 mpg was about 3 mpg better than my normal (lightly loaded (weight) and standard drag profile) . Lower speed and tonneau did make a difference.

2002, 2500HD, 4wd, extended
Stock Duramax/5 speed Allison
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Old 05-30-2006, 10:07 PM
REDTRUCK05 REDTRUCK05 is offline
Join Date: Aug 2005
Posts: 126
Default 05 Lly Owner

Originally Posted by Stlheadake
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!)
I own a 05 LLY 2500 HD crew cab. My MPG pulling 5600 LBS at 65 mph has been between 9 and 11 mpg. the cooling fan runs at any temp above 70 F.
2007 GMC 2500 crew cab LBZ
6.6L duramax
Roof markers and XM radio
Rhino liner
Plat series nurf bars
Putco bed rails
Truxedo bed cover
edge juice W/ monitor temp uninstalled.
Hypertech stage 3
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Old 05-31-2006, 01:02 PM
jharden1 jharden1 is offline
Join Date: Mar 2006
Posts: 44

'04 1/2 LLY/Allison 4Door short bed. First tank got 17.4 MPG. I now have 32k on the odometer. 42 miles each way to work. I run 68-70 on the cruise control. 19.4 calculated last tank. Tire pressure 60 all 4 wheels except when towing, then 80 on the rear.

8000# travel trailer, 4 adults in truck 500# in bed, 11.3 calculated at 65-70 on cruise, in west Pa.

After initial breakin in I averaged 18.4. I recently had EMC reflashed due to smoke at idle. Fuel mileage is up a little, say .2mpg.
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Old 05-31-2006, 01:46 PM
Mark Rinker's Avatar
Mark Rinker Mark Rinker is offline
Join Date: Jun 2003
Posts: 5,813

Also, I would hand calculate your fuel mileage as a longterm average over weeks and months, not single tankfuls. A spreadsheet is a great way to do this. Just a moments worth of data entry, or enter the information off your fuel card statement each month. My Holiday and Shell cards record mileage at the pump at the fillup, so its really easy to do by the month. I know when a particular trucks performance has changed, usually a dirty air filter is to blame.

You'll soon settle into a number that will be noticable when you change something like intake/exhaust, etc. When you complete the mod, start another longterm average over a number of weeks and tankfuls. Then and only then will you know if you have truly picked up a solid increase in MPG.
2011 Chevrolet Tahoe 5.3L daily driver
  • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW, '05 Denali
  • Total GM diesel miles to date : ~950K
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Old 06-01-2006, 12:18 PM
Finman Finman is offline
Join Date: May 2006
Posts: 8
Default 2005 DMax 2500 Crew

I ran a trip Minnesota to Kentucky. A hair under 1900 miles all freeway cruse on 70-75mph. I have a Hypertech tuner and ran it in stage 3. 17.4 MPG hand calculated. Not impressed. Would have got 21-22 with my 02. Black smoke when you put the wood to it and alot of it. Anyone have any thoughts on that? Does not seem to smoke when running in stock.
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Old 06-01-2006, 02:55 PM
Mark Rinker's Avatar
Mark Rinker Mark Rinker is offline
Join Date: Jun 2003
Posts: 5,813

My Hypertech tuner is great at adding fuel (power) but not so great at adding enough boost to burn all that extra fuel (black smoke).

If I were running down the road unloaded and wanted max mileage, I'd either add enough boost to burn the extra fuel, or return the truck to stock settings for the trip.

Your speed of 70-75 has alot to do with it. In fact, I'm a bit suprised that your LLY did that well!

Next trip try stock settings and see how you do, or call JK and add a BoostStick and enjoy the extra (although probably unused) power...
2011 Chevrolet Tahoe 5.3L daily driver
  • Previous owner of two 1994 6.5L K3500s, '01, '02, and '05 6.6L K2500s, '04 C4500, '06 K3500 dually, '06 K3500 SRW, '09 K3500HD SRW, '05 Denali
  • Total GM diesel miles to date : ~950K
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Old 06-02-2006, 04:20 AM
ToddMeister's Avatar
ToddMeister ToddMeister is offline
Join Date: Feb 2000
Location: Michigan
Posts: 358
Default 20 mpg is possible at 70mph

This past winter I made some trips to snowmobile and got 20mpg at 70 mph hand calculated (on winterized fuel, too). I filled up the truck, hit the expressway, drove a couple hundred miles to our destination, and filled truck up. I did this 3 or 4 times. No fibbing here...

Originally Posted by David Proske
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.
2016 GMC SLT 2500 CC 4x4 Duramax All Terrain 18K
1995 GMC 2500 EC 4x4 6.5TD,2138,Custom Tuning,HX40W-II turbo,Amsoil Bypass Filter,KD Exhaust,FSD Cooler,KD Headlight booster,Hi-cap Cooling
TDP member #14
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Old 06-02-2006, 06:42 AM
Kennedy Kennedy is offline
Long Distance Crew Chief for BMDMAX
Join Date: Mar 2000
Location: Loyal WI US
Posts: 10,691

I can't get 20's at 70 mph, but at 60-ish mph I can run solidly into the 20's. It will be interesting to see what Uncle Wally reports as I just sent him a tune for his dually. The DIC shows encouraging results so far, just need to hand check to verify...
Kennedy Diesel-owner
More than just a salesman-I use and test the products that I sell on a daily basis!
Superflow Lie Detector in house
2002 Chev K2500HD D/A CC Long LT 11.77@ 124mph at 7700# fuel only-e.t. needs help
2005 Chev K3500SRW D/A CC Long LT(SOLD)
2007 Chev K2500 Classic EC Short LT (Sold)
2012 GMC K3500SRW D/A CC Long LTZ Happy Birthday to me! Built 1 working day after my birthday and delivered 7 days later.
2016 GMC K3500SRW D/A CC short LTZ

Custom tuning in house using EFI Live tuning software!
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Old 06-02-2006, 10:31 AM
Uncle Wally Uncle Wally is offline
Join Date: Nov 2002
Location: Howard Lake, MN
Posts: 381

Just to give a comparison as to what my fuel mileage was before John's tune.

05 LLY Crew Cab Dually - stock tires.

Loaded - light to moderate

Modifications at the time were John's 4" exhaust, Uni Air Filter, Mega Filter and Lift Pump/Fuel Pump Pickup mods. I had EFI Live at the time, but chose to leave the stock tune in it for a while.

I was pulling a two place snowmobile trailer with two snowmobiles on it and I had one snowmobile in the bed. There were three of us and our gear in the truck. We added two honda three wheelers to the trailer on the way back.

I drove from Minneapolis to Island Park Idaho in March this year. Temperatures were in the mid 40's for most of the trip out and in the low 60's for most of the trip back.

My best fuel mileage per tank - hand calculated - was 12.5. That was treating the throttle like an egg, using cruise at 60 and never letting the tack go over 2000 rpm.

My worst fuel mileage per tank - hand calculated - was 10. My personal belief is that I got a batch of marginal fuel. I didn't change my driving style much, so I don't know of any other explanation.

Unloaded - with tonneau cover - EFI Live now installed - only changes to the tune were to EGR controls. No added fuel, no altered timing.

I get my best mileage at 50-52 mph. Treating the throttle like an egg, never letting the tack over 2000 rpm even on acceleration. I can get between 16 and 18 mpg. I drove this entire tank of fuel - no wife variables on this one.

I usually don't drive that way day to day, nor does my wife. Normal driving yields a pretty consistant 14-15 mpg.

The only issue is that I spoiled my wife with the new truck, and she won't drive our gas miser car anymore. My wife drives totally different than I do even with me giving her constant grief. Her fuel miser skills have gotten better, but are still not at my level. She only understands two pedal postions: foot completely off the pedal, and the pedal all the way to floor (for both foot feed and brake pedals). With that said, it's hard to get a good fuel economy run when my wife drives. I will work on that when we head to Pittsburgh in July. That way I can actually show her what I am talking about.

Sorry for the long post, but I thought it would be good for a comparison. I installed John's tune yesterday and drove about 45 miles on it. Fuel mileage on the DIC was astonishing. I won't give numbers at this point as I need to hand calc it. You'll here from me after a couple tanks of fuel.

05 K3500 D/A Crew - Sport Red and sexy! JK exhaust and filtration, guages and EFI Live. What a blast!

71 K20 awaiting Cummins 12v swaps

For Sale: Deep Sump Trans Pan for a 4L80E
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