2004.5+ LLY Duramax Fuel Economy Poll

[Chuck1]

I have been getting 20 to 22 mpg highway and with my 5th on 15 .Its only a 25footer and i can hardly tell its on there .Man I love this truck .I had a 6.5 before this for 10 years never had a problem did a few adds. thermestat high out put water pump and fan I never had it overheat and i live in some major mountians,I tow quite a bit flat deck trailers and my 5th but after reading about it here. I thought best to do the cooling mods. It was a good truck also when I sold it the buyer said what dosn't work on it ? It was a 94 and had a full load and I thought for a minute and realized every thing worked and had never given me a problem.It was a little slower up the hills than the dmax but a really good truck.If my 05 dmax treats me near as well I.ll be happy. And boy with the extra power and the heated seats I'm in heaven !

[Londell]

I have an 06LLY with 26,000 miles no problems with anything so far. Get 24mpg on highway at 55mph at 60mph get 22mpg at 65mph get 21mpg at 70mph get 19mpg. Mixed driving get 18.5mpg. I tow a 36foot fifth wheel that weighs 14,000 lbs fully loaded and get 10-12mpg depending on grade over here in California. Not bad I say.

[Truck Specialties]

Alot of people believe that a programmer, intake and exhaust are all you can do to optimize fuel economy. Just the tip of the ice berg. The LLY has huge potential, in our eyes it gets the best miliage of all years. I just got back from a 3,000 mile trip with my 05 crew cab dually with 285-75-16's AT's(you know the tires everyone says hurts your milieage) I pulled a tall horse trailer bumper pull, weighs 6500 lbs. for 1500 miles Drove 80mph the entire way, averaged 12.5 mpg. Turned around empty drove 80 to 85mph back home, averaged 22.9mpg calculated not relying on the dash. Speedo is corrected for tire size. Gains come from small things all adding up. Stay with programmers, stay away from tuners like Banks and Edge if you want miliage. Get rid of cats and egr systems that flood the motor with garbage air, open up the intake elbow by the turbo, seal up the radiator and intercooler to force the air through, electric fans are the tits! Water/meth injection will keep the miliage up while towing and egts down. Remember black smoke maybe cool, but it won't make power.

[dannomatt]

2005 C/C L/B SRW 2WD LLY. 37,000 miles.
Back when it still had the 245s on it, I got a real 18-20, with a best of 20.8. (I drive old...1800 - 2000 RPM)
I put 265s on it, and I had the speedo re-calibrated @ Santa Paula (Ca.) Chevrolet. (Excellent Svc Dept)
I promptly dropped to 15-17, with the best being 18.
I feared something besides the speedo had been programmed, so I called the Service Manager. Sensing that the tires were the culprit, he gave me a golden opportunity: They temporarily reinstalled a set of take-off 245s, re-calibrated, and turned me loose. 250 miles later, I was back at 20 MPG. (Of course I had them put my 265s back on - They look better!)
All the MPG numbers are figured old-school - No DIC.

So. Where-oh-where do I go to get that fuel economy back? The local diesel shop recommends the Diablo Sport programmer. As good as 2-4+ MPG is what his customers claim they picked up. He says don't bother with intake/exhaust mods.
At 1800 RPM, cruise control on the freeway, I'm a consistent 17-18, and I know that's not bad...But if there's a little better to be had, I'd be interested in hearing about it.
Thanks

[DmaxMaverick]

Welcome aboard!

Looks like your mileage didn't change, much. Your dealer didn't (and can't) reprogram/correct the speedo/odo. The changes they make in the PCM ONLY correct for ABS operation. The tire change increased your roll-out by about 5%, which corrects your fuel mileage almost back to where it was before, but not quite. While taller tires offer a potential for an economy improvement, wider tread reduces it (for a number of reasons). Switching to a 235/85/16(32") or 255/85/16(33") offer the best opportunity for an increase.

Power programmers won't improve your mileage, generally. They may offer the opportunity to help you improve it, but ultimately, it will be your driving style and habits that will determine the actual outcome. The Predator programmer (Diablosport) will allow you to correct the speedo/odo for tire size changes and increase power, but won't help fuel mileage by itself. Only your right foot will do that. Normally, having more power on tap temps one to use it, which will in every case, decreases mileage. The advantage of having more power allows you to drive under the same conditions in a higher gear (under load or ascending a grade), which can increase economy, provided you resist the temptation to use the additional power. If you get/use any power programmer, be sure to also install, at the very least, an EGT (Exhaust Gas Temp) gage. A boost gage is also recommended, but the EGT gage is necessary to monitor the exhaust temp, which can cause sever engine damage if exceeded, or prolonged.

[dannomatt]

Thanks. I'll continue researching. It seems that gauges are are universally recommended to go along with most modifications. Understood.
It's frustrating how many opinions there are regarding the re-programming of a vehicle's ECM.
The dealer who re-flashed mine insists that all they did was correct for tire size. (They also helped me determine that the tires were the main culprit in the loss of MPG)
The local diesel mechanic says the dealer unavoidably gave me some EPA / AQMD 'updates' that are probably causing additional fuel consumption. (Dealer insists that only speedo 'files' in ECM were opened)
A GM rep told me that the only ECM re-flash they support is not related to tire circumference. He even went as far as saying "...there are some parts of your ECM that are better left un-touched."
The 2-4 MPG loss isn't killing me, especially considering that I know the tires are most of it. My goal now is to determine if there are mods that can some of it back.
Great truck otherwise!
Dan

[DmaxMaverick]

Some mods can help improve your mileage. Problem is, at what cost? The reality is you may not see an ROI (Return On Investment) until 100-200K miles or more, or never (depends on how much you spend, the mileage increase, and how long/far you keep/drive the vehicle). Driving habit adjustments are free. However, in every case, YMMV.

[Diesel 4me]

Wishing that mileage could be improve don my Dmax as well. I have a 2005 with aprox 22,000 miles - stock sized tires and my DIC records aprox 13.8 while driving down the highway at 60 (Ca).... Engine stock for now ... no programmer

[JohnC]

Don't believe the DIC unless you have confirmed its accuracy. Calculate it based on mileage and fuel purchases. (Verify odometer accuracy, too....)

One easy way to check the accuracy of the DIC is to zero the fuel used when you fill up and see if it agrees with your next fill. Repeat a few times to get a feel for its accuracy.

[asm]

I voted less less then 15MPG.

My Duramax is a 2004 Dually with crew cab. When towing a 11000# fifth wheel, going up 395 to Bishop in speed between 35MPH to 55MPH, I get 9.8MPG. 35MPH is not by choice, as LLY is known to overheat when push hard going up a long grade.

I got my best MPG yesterday going from Bakersfield to Orange county to drop off a trailer. The trailer weight about 7000# and I kept the speed between 55 ~ 65MPH. Of course, the returen trip was driven with nothing in the back of the truck. I actually got 14.1MPG. Not as high as some of the 20MPG claim I see here, but much better than 10MPG I used to seeing.

[AKMark]

05 LLY, CC, LB 94K miles, 15K hours, unknown age of injectors, runs great though.

In Alaska with temps in the -20 range, 15 driving approx 14 miles one way twice a day every day. (56 mph, if I drive 60 this drops to 13)

17 over a 85 mile drive with 3 hills over 8 degrees of upward angle. (67 mph)

This is with #1 Diesel, mixed with 2 stroke oil.

I haven't taken it on any longer road trips yet, and unfortunately all of our roads out of Fairbanks will encounter big hills.

[gary_lucas]

Had the truck for almost a year now, put about 12k on it.

Overall thinking it's a great truck, but the fuel economy is less than I hoped for.

My last set of numbers put me in the 14.2mpg range with mostly city driving unloaded.

When I got the truck last year it had:
* Modified exhaust (Not sure what it is, going to find out tomorrow).
* Edge Juice with Attitude.
* p265/75's

I've removed the attitude module this evening for the first time. I had a bakflip installed this past week and I'm going to get a Kennedy tuned ECM for our trip down to Las Vegas next month. We'll be hauling a 3200lb trailer so I'll get a good idea of the mileage at that point.

My understanding of how to drive with fuel efficiency in mind basically starts and ends at takeing it easy, don't floor it to get up to speed, Keep it at the low end of the speed spectrum (IE on the highway keep it towards 55 rather than 70 if at all possible...)

When the current 265's start getting old I'll probably replace them with the stock 245's and hope to see some better fuel economy.

[gary_lucas]

Followup, just drove from Vancouver BC to Las Vegas Nevada and back up the I-5 in California.

Combined average of about 17.5MPG the whole way. I was towing a 3000lb trailer with a really low profile (low wind resistance).

I was getting close to 19MPG on the I-5 on the way back, going through the mountains brought it down to 17.5 approximately.

[AKMark]

First long road trip with my 05 this last weekend.

734 miles driven (per gps). I have an 80 gallon external tank in the bed that keeps the main topped off. It took 35.7 gallons to top off the external after we got home and I confirmed the main was full.

20.7 mpg, I drove 67mph (per gps) except for about 200 miles on the way back when I stepped it up to 72mph to keep up with a convoy of friends.

Considering my motor has over 16K hours on it, I'm happy.

[dfenze]

04.5 LLY with 110K miles. When new I was getting 18-19, no matter what I did. now, I get around 15-17. I have some upgrades, but I am also having a lot of issues with the motor right now.

[AKMark]

Tried a different fueling place. Glad I did.

Running short trips (14-18 miles at a time usually) I'm averaging 18-20 with summer weight fuel and a little bit of 2-cycle oil to give me some lubrication.

I haven't had a long road trip since using this stuff, and my bad luck I will be back to winter fuel before long. Summer doesn't last long here.

I found two of the other stations I used to fill at run winter fuel all year round. Jerks.

[Rhynno]

I have had my 2005 GMC 2500HD Duramax for over a year now, and it gets horrible fuel mileage. It is lifted with 35" tires, has an aftermarket air intake system, Edge Attitude chip, and aftermarket exhaust (not sure on the gears, assuming stock). I get between 10.5 and 11.5 MPG. At first, I thought it was the chip because it was set to Stage 3 and had tons of power, so I changed it to Stage 1, the economy setting. I only gained .5 MPG, so I changed it back to Stage 3 to get the 70hp and the 120 Ft. lbs. of torque. I also have a 2000 Ford Excursion 7.3 Powerstroke which I have owned for 14 years. It is much heavier than my Duramax, especially when the 44-gallon fuel tank is full. It has a 6-inch lift, is running 37" BFG KM2 M/T tires, and the gears are stock. For the life of the Excursion, I have averaged 17.3 MPG around town and I have even been able to achieve 20.4 on the highway, loaded down with camping gear and people. The big difference I see between these vehicles is the Excursion has hubs and the Sierra doesn't. I haven't had a lot of time to work on the Sierra, so I am sure I will be able to tweak it some once I can spend a little time and money on it, but I was hoping it would at least deliver 15 MPG around town. I hope to get one of the hub conversion kits from the Ram Man this year and get that installed to see what difference that makes, but at this point, I have just done routine maintenance. I will also give it a tuneup when time allows. Love the truck though, drives awesome, very comfortable and get amenities!

[wmholden]

In 2005 I averaged 21.7 mpg on a trip from Texas to Savannah @ 67.5mph. Last week I averaged 18.5 over an 800 mile trip, half of which include about 1200 lbs of hickory in the bed and 70-75mph. So I have no complaints. I love my LLY. And since I now live in town(again)my city mileage is 13.5 to 13.9 mpg. We are looking at the DT400 performance package now. We are approaching 160K and are ready for mods.
DT400 components for under $800 total look like a nice foundation for performance- now and future. I hope to find reviews on the DT400 kits