2004.5+ LLY Duramax Fuel Economy Poll

[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.