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Chevy Silverado and GMC Sierra Duramax Fuel Mileage

I just bought a new 2004 Duramax Chevy, extended cab with short box. I filled the tank at 226 miles and showed 17.2 mpg. I filled again at 500 miles and now show 15.2 mpg. This is kinda discouraging. Is this normal? I bought the D/MAX expecting 18-20 mpg. The only difference is a tonneau cover being added between fill-ups. I drive 25 miles to work on four lane highway at approx. 70mph. Any comments??


  • gagricegagrice San DiegoPosts: 28,850
    I got real excited about the DuraMAX when it came out. I read so many cases of mileage in the range you are talking about that I lost interest. I was hoping for 24 mpg on the highway and have not read of anyone getting that.
  • The worst tank I ever got was 17, and the best was 22. Overall average between 18-19, city and highway.

    But...manual transmission (6 spd), and lightweight regular cab.

    And the new LLY is a completely new design, new heads, new injectors, new garrett turbocharger.
  • gagricegagrice San DiegoPosts: 28,850
    And the new LLY is a completely new design, new heads, new injectors, new garrett turbocharger

    Is that the new GM DuraMax? Is it a 2005 model engine? How is the power compared to the older models?
  • The LLY was released in the 2nd 1/2 of 2004 year model run. It has more horsepower and torque, by exactly how much...I forget, something like 320 hp and 590 lb/ft torque, give or take a little.

    The hp and torque increase is for Allison 5 speed automatic equipped models only, not the ZF-6spd manual.

    The hp and torque of the previous LB7 was 300 hp, 520 lb/ft torque.
  • ...of the new Duramax LLY are the new heads, which put the injectors *outside* the valve covers, rather than underneath them as on the previous LB7. This is important because if an injector leaks, it will spill diesel fuel down the outside of the engine, rather than cause the crankcase to fill-up and overflow from the inside.

    Unfortunately, that's exactly what's been happening. GM recently sent out notices to owners of 2001-2002 Duramax diesels that they *may* experience injector failure caused by high fuel return rates or cracked injector bodies, and that accordingly, GM will extend the warranty on *covered* failures (not all) to 7 yrs/200,000 miles. While that's a small positive step, my $0.02 is that GM has not gone far enough. Since they KNOW they have a problem with the Bosch injectors, the appropriate and ethical course of action would be to issue a recall for all affected vehicles, and replace the bad injectors with the updated ones the owners should have expected in the first place. FYI, these injectors operate under very high pressures, up to 30,000 psi, making them completely different animals than cheap injectors in gasoline engines which operate at a lowly 50 psi, and cost less than $100 each. The bosch diesel injectors cost upwards of $500 each, 8-12 hours labor to replace them, and some have been replaced 8 at a time under the GM warranty. You do the math...that's quite a bit of exposure for an owner to accept. Diesels are fun, but you could pay to play if you hold on to one beyond the warranty period. The bosch injectors are on national backorder at this time. Dealers cannot give a time frame if you are unfortunate enough to suffer this malady, except first come-first serve.

    The other features behind the new heads and variable inlet garrett turbo-charger are primarily to address emission concerns and future low sulfur diesel fuels.
  • gagricegagrice San DiegoPosts: 28,850
    That is good info. I was thinking of a used Duramax. Gives me reason to hesitate. Is there a problem if you do run the ULSD in the Duramax? You know I was so excited when they came out with that engine. I have held off buying a big truck to have it all sorted out. Now I don't know if I should go for the new Cummins or Powerstroke instead. I hate the looks of the Ford and Dodge.
  • I guees I will just have to ride it out and see what happens. I have friends who say they are getting 18-20 mpg. I have one friend that has the equipment to re-program the computer, says it gave him 2 more mpg. Any comments on this? First diesel for me, I'm green.
  • No reprogramming on mine...stock all the way. The quality of fuel can make a difference, also summer blends or #2 versus winter #1, which is thinner so as not to gel in the cold.

    The cetane rating of fuel affects mileage and power, but it's hard get reliable information, or even know what you are getting at the pump. Find a reputable fuel source that you trust, and stick with it.

    I think your mileage will get better, however if you have the newer LLY design, some reports are that it hasn't been getting as good a fuel mileage. It makes more horsepower, so the re-programming that matters most is possibly located underneath your right foot.
  • 2001 2500HD,6.0, Automatic transmission. Just bought used. When accelerating truck feels like its under load. Is this normal for this truck? How does yours feel. Also, seems like there is slight slip when going from a dead stop to start. You can feel it a little.
  • Thanks for the info. How would I know if I have the LLY design? Is it on the new car window sticker or do I need to find an engine number??? Most of the Cetane I have found is 40, I hear there is a place around town that sells 44, I will check it out. Last tank got 15.8. Thanks for the info.
  • I don't know how you tell, unless it's in the VIN or engine code or something, or maybe on the certificate of origin or titling papers...

    The old design is the LB7, new design is LLY, a mid-year 2004 introduction. The LLY has more horsepower and torque too, and apparently worse fuel mileage...but again, that could be a consequence of the driver.
  • Or, if you see something that looks like injectors on the outside of the valve cover, because the LB7 design has the injectors UNDERNEATH the valve covers, so you can see only the fuel rails.
  • Found an easy way to tell if you have the LB7 or the LLY that came out in mid 2004. The owners manual shows oil filler caps for each engine. The oil filler cap for the LB7 has the following: (Engine Oil, 15W-40W.) The oil filler cap for the LLY has the following: (Engine Oil, HUILE-MOTEUR.) When I raised the hood I saw that mine was the LLY. I understand that these engines do not get the fuel economy that the old style gets. Also, I ran a tank of fuel through with the tonneau cover rolled up and went from 15.8 to 16.8. I am now runing another tank through with the cover in place. I'll see if that really makes a difference.
  • ezshift5ezshift5 West coastPosts: 853
    blackMax, how about that fuel usage?
  • Fuel mileage dropped back to 16.0 with the cover on. BUT! I also found out that my tires only had 40psi in the front and 70 psi in the back. I have gauged them up to recommended 80psi and I am running another tank through. Will let you know.
  • youngeryounger Posts: 16
    2004 Crew short bed. I have been getting anywhere from 17-19.....current odometer: 2500 miles. Not too bad with a topper on the back. I'm guessing the mileage will get a little better once the miles start to season the engine.
  • Though I have now sworn off Duramax trucks, I have owned two of them, an '01 and an '03. I never got any better than about 15mpg average with either one. If you drive them no faster than 65 mph you can do better. I was making trips out west where 80+ mph highway cruising speed is the norm. There the D-max returned about 13-14mpg empty.

    I have three Cummins powered Dodges, two '03s and now an '05. the '03s will cruise at 80--90 and knock down 20 mpg empty. They are also quieter and don't have an impending injector failure in their future.
  • 3 weeks ago, I just bought a 2005 Crew Cab Short Bed Duramax Diesel.. First off, I love this truck. I traded in my 2001 1500 ext cab. Need the diesel to tow my horses and the crew cab to put the kids in, needed more room..


    I have been on a few long trips, I just got back for a long trip 400 miles round trip some highway and some city driving the last part of the trip was all up hill going from sea level to 7,000 feet. I got around 20-21 miles per gallon average speed around 80 to 65 some traffic with stop and go. Very happy with the mileage I got. Much better than my 1500, which I got around 19 or less on highway, and forget about climbing up the hills it was a dog, and towing it did good sense I was maxing out the weight limit.. Glad GM made changed this model year... Love my new truck
  • chanichani Posts: 1
    I'm looking at buying my first diesel truck purchase to haul primarily, a 3 horse trailer, and for standard road commuting, not heavy hauling. We started out looking at F250's, but are also looking at the Chevy's since we don't pull really heavy loads, the incentives are attractive, and the cab has a better feel for me, since I'm pretty small; and the rig and bed are lower to the ground.

     I have some concern over the engine reliability,however. Any comments out there on the new Duramax engines?

     My biggest concerns, of course are engine and transmission reliability-- I don't want to get stuck out on the road with horses.

      Next is handling and safety; I'm not an experienced "big truck" driver. Turning radius is a factor, and the new tow package on the F 250's is an attractive feature.
  • rooster9rooster9 Posts: 239
    The easiest way I found to find out if you have the older LB7 engine or the newer generation LLY engine is by looking at the vin number. If you have a Duramax diesel, the 8th number in the vin will either be a 1 or a 2. 1 is LB7, 2 is LLY. rooster
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