Hauling Stuff / Diesel Highway Mileage

jasonvp · April 20, 2005

Hey folks -

First, I'd like to share an interesting (for me) experience. My 2500HD Duramax is the first "big" truck I've ever owned. This weekend, a buddy of mine asked me to put the truck through a test. He had a bunch of old lumber in his back yard that had to be taken to the dump.

We almost over-filled the bed with old, water-logged lumber, and off we went. I was quite amazed at how docile the truck was, even with all that wood in the back. No problems accelerating from stops, or coming to a stop. It steered and manuvered just as if it were empty.

We got to the dump, and the truck checked in at over 8300lbs. With a curb weight of 5900lbs, that means I was carrying over a ton of stuff in the bed (along with the fuel weight, the oil, other engine fluids, plus the two occupants.) This may not be amazing to some of you guys that haul stuff on a daily basis, but I was quite shocked at how well the truck handled the weight. Woo hoo!

Now, to a question: my DMax is still breaking in. It has just north of 600mi on it so far. I filled it up, reset the fuel mileage computer, and set out on a highway trip on Sunday. I was averaging a cool 80MPH down I95 (through traffic,) and was seeing low 17s for mileage. That was with the windows closed and the climate control on. On the way back, I opened the windows and sunroof, and shut the CC off. I saw high 16s. I closed the truck back up, turned the CC back on, and the mileage improved to low-mid 17s.

Given the engine's age, am I to expect better on the highway once it breaks itself in? Or is 17 about the limit?

Thanks!

jas

snoman · April 20, 2005

It may improve some but at 80 mph that is good MPG and it would likely be closer to 20 mpg or so at 65 as drag and HP needed to move truck increases greatly as speed increases above 65.

Truckman031 · April 20, 2005

That is great mileage for a diesel with that mileage. Diesels can take up to 25,000 miles to fully break in. As the engine gets more miles, you will probably see an increase in mileage, but I don't know how much.

dewfpo · April 20, 2005

Keep the windows closed, the CC on and slow down. You'll be impressed.

DEWFPO

jasonvp · April 20, 2005

Keep the windows closed, the CC on and slow down. You'll be impressed.

I don't understand this phrase which you speak: slow down? :-)

No doubt, the aerodymanics of the truck (ie, a rolling brick) aren't helping with the mileage. And it was clear the drag created by the open windows and sunroof hurt. Further, the engine is reving close to 2500RPMs at 80MPH. So, yes, going slower would lower the RPMs and thereby improve the mileage immediately.

I was just curious if I should also expect it to improve with time. It sounds like I might see a small improvement, but nothing earth-shattering.

Thanks!

jas

snoman · April 20, 2005

Keep the windows closed, the CC on and slow down. You'll be impressed.

I don't understand this phrase which you speak: slow down? :-)

No doubt, the aerodymanics of the truck (ie, a rolling brick) aren't helping with the mileage. And it was clear the drag created by the open windows and sunroof hurt. Further, the engine is reving close to 2500RPMs at 80MPH. So, yes, going slower would lower the RPMs and thereby improve the mileage immediately.

I was just curious if I should also expect it to improve with time. It sounds like I might see a small improvement, but nothing earth-shattering.

Thanks!

jas

<{POST_SNAPBACK}>

It is not because the RPM is decreasing in as much as the amout of HP it takes to power the truck at a higher speed drag wise than a lower one so it takes less fuel per mile and yeild greater MPG. Trucks do not operate in a vacum with no rolling resistance to fuel consumption will go up with speed regardless of RPM.

Mark_in_Tulsa · April 20, 2005

A Duramax HD truck as a curb weight of 5900??

ToBNamedL8r · April 20, 2005

Sounds like I'm in the same boat. I just passed 600 and will be taking my first long highway trip next week. I am anxious to see what kind of mileage I get. I'm getting about 13.5 in town.

wpatrus · April 20, 2005

I am in the same boat as you guys are. I traded in my 6.0 HD for a new crew cab Duramax/ Allison. And I love it. I had it out on the highway and back roads last weekend and the DIC showed 20.5. Now going from my 6 liter with 12 miles per gallon to 20.5 almost gave me a heart attack. I am gonna love this.

I also added a Diablo Preditor to the truck, amazing power and it only helps the milage (with driver input of course). Now I am only waiting to see how much I might get it to climb, this is great. . .

Jpshostr · April 20, 2005

With diesels it seems 2000-2100 RPM is the magic engine speed, no matter if its a Duramax, Cummins, or Powerstroke. Stay at it or below you'll get good, acceptable milage, go above and it'll go in the tank. I've found wind drag through open windows or from running the AC to make a negligable difference in mileage, maybe a tenth of a mpg.

With my Cummins, 48RE tranny, and 3.73 rear end, 71mph puts me about 1950-2000RPM, so thats where I set cruise. I get about 19 mpg on the flat interstate there. Jack up the speed so I'm running the engine in the 2300rpm range, she tanks to 15mpg.

Kid at work got a new Duramax about the same time I got my Cummins, same deal there. He has quite a lead foot, running about 80mph all the time. On his trips home to OK and back on the weekends he was getting about 13mpg, pretty piss poor. I asked him for curiosity sake to drive awhile around 72mph or so and get those RPMs down, milage went up to 15.5-16 for that stretch.

My service truck is an 02 Ford with the 7.3L Stroke, 4x4, auto, the whole bit. Since I just fill it up with the company card when it needs it I don't keep track of mileage all that closely, but over my trips I've noticed that when I'm running in Iowa, Illinois, or states to the east where I'm at a 65mph speed limit, running 70mph, I get a lot more miles out of a tank then I do when in the west, such as Nebraska, Wyoming, Idaho, Dakotas, where I'm doing 75-80mph. Granted, its very heavily loaded, especially with it being a SRW, but I'm in the 15-16mpg range. The 6.0L that can be directly compared to it in our stable gets a consistent 14mpg.

Diesels are work engines, not speed engines. If you wanted to go fast, you screwed up with your engine choice, no doubt on that.

As for the weights, something doesn't seem to jive here...truck seems awful light empty, if its correct I'm even happier I jumped ship from GM as lightly as they're built. Hell my Dodge weighs in closer to your "full" weight when its empty, in the upper 7Ks. Not gonna touch the bed weight you were carrying either, those that have been on this forum since the beginning know what I'd say to that.

Shaners · April 20, 2005

I put 250 miles on mine last weekend. About 110 on the Interstate, about the same on 2-lane roads, and about 30 miles putting through the mountains on dirt roads searching fro the ultimate trout stream.

I averaged 17.5 MPG's according to the DIC.

The truck now has 630 miles on it as it sits in the garage tonight.

jasonvp · April 21, 2005

Thanks for the input. Very interesting data you provided.

Diesels are work engines, not speed engines. If you wanted to go fast, you screwed up with your engine choice, no doubt on that.

No screw-up at all. Humming along at 80MPH on I95 (and I was being passed, by the way..) didn't seem to be working the engine very much. It was spinning at 2500RPMs or so, but it seemed to be quite happy.

Be interesting to compare the mileage of the same truck moving at the same speed, with the 8.1L in it. But, I specifically didn't want this to turn into another gasoline vs. diesel argument. That wasn't my point.

As for the weights, something doesn't seem to jive here...truck seems awful light empty

I offer GMC's website as the source of my weight numbers:

http://www.gmc.com/specs/specs_content_fil...&vehicle=2500hd

Look at the table at the bottom of the page. The 4WD Crew Cab/Short Box truck (which is what I have) checks in at a curb weight of 5858.

jas

Shawn T · April 21, 2005

Thanks for the input. Very interesting data you provided.

Diesels are work engines, not speed engines. If you wanted to go fast, you screwed up with your engine choice, no doubt on that.

No screw-up at all. Humming along at 80MPH on I95 (and I was being passed, by the way..) didn't seem to be working the engine very much. It was spinning at 2500RPMs or so, but it seemed to be quite happy.

Be interesting to compare the mileage of the same truck moving at the same speed, with the 8.1L in it. But, I specifically didn't want this to turn into another gasoline vs. diesel argument. That wasn't my point.

As for the weights, something doesn't seem to jive here...truck seems awful light empty

I offer GMC's website as the source of my weight numbers:

http://www.gmc.com/specs/specs_content_fil...&vehicle=2500hd

Look at the table at the bottom of the page. The 4WD Crew Cab/Short Box truck (which is what I have) checks in at a curb weight of 5858.

jas

<{POST_SNAPBACK}>

That curb weight could be a 1500HD with a 6.0L in it. The good money says you will have to add another ~800lbs for the HD with the Duramax, Allison, wider aft frame, and heavier rear axle. That's just my guestimate, could be more, could be less.

jasonvp · April 21, 2005

That curb weight could be a 1500HD with a 6.0L in it. The good money says you will have to add another ~800lbs for the HD with the Duramax, Allison, wider aft frame, and heavier rear axle. That's just my guestimate, could be more, could be less.

That web page is specifically for the 2500HD truck. The Duramax and Allison may weigh in more, but not enough to make a huge difference in the truck's overall curb weight.

jas

KyleBst264 · April 21, 2005

Yesterday on a 44 miles round trip that was mostly highway, I averaged 19.7 MPG. I was going on average about 70-75 MPH.

When I tow my trailer (7,200 lbs, shown in sig pic) I get an average of in between 12.8 and 13.2 MPG, this is on almost level terrain and cruise control set on on 72 MPH the whole way.

When I go on moutain roads with the trailer it'll get about 11.7- 12 MPG. This is with no cruise control, and the driver planning for the hills up ahead.

Right now, buying diesel fuel sucks. It used to be alot cheaper than gasoline when I bought my truck last year. Still, I have no regrets in buying the Duramax, the 2500hd is a hard-worker that doesn't break a sweat.