Mileage for gas trucks vs diesel trucks?

[Sky]

Originally Posted by BillTex

OK-found one. It would appear that fuel consumption was greater at peak torque. This would make sense.

Interesting.

Bill

Bill, maybe I am miss understanding you, but the best mpg you can get in a vacuum is when the engine is at peak torque. Add in air resistance, and it is a little below peak torque. On a DuraMax 6.6 with Allison, thats about 1500 - 1600 rpm or 55 to 60 mph.

Sorry, but I don't believe your real world data . You sure your DIC is accurate and the terrain was the same each test?

Dale

[BillTex]

Originally Posted by Sky

Bill, maybe I am miss understanding you, but the best mpg you can get in a vacuum is when the engine is at peak torque. Add in air resistance, and it is a little below peak torque. On a DuraMax 6.6 with Allison, thats about 1500 - 1600 rpm or 55 to 60 mph.

Sorry, but I don't believe your real world data . You sure your DIC is accurate and the terrain was the same each test?

Dale

Dale, I am not sure you will get peak efficiency at peak power (tq). You could also assume peak output (tq) would require maximum fuel consumption. If I had more time I could have tried the test on either side of 1600 rpm (max tq output for this Duramax).
But as you also noted, my peak efficiency was "a little below peak torque". So what's not to believe?

The DIC is accurate, I have verified by calculation many times.

One big variable (I did say this wasn't very scientific-just some observations from this past weekend) is elevation, as you noted. The tests were all done on the same stretch of highway, in Vermont and NH, but I did not have an altimeter with me so could not measure total elevation (gain/loss) for each 1/2 hour leg of the test. Anecdotal observations were that elevation gain/loss were about the same (i.e. I did not go from the mountains to the prairies durring the test).

I was stumped by the numbers also, if I had more time (traffic patterns began to change and no longer would make for a "fair" comparison.) I would have repeated the 65 mph leg.


Do you have any data on peak efficiency of the Duramax?

Thanks, Bill

[Sky]

Hi Bill,
I swear the "Brake Specific Fuel Consumption" link once worked on this page, but it no longer works for me. Maybe you will have better luck. Here it is.
TheDieselPage.com Feature Articles

Here is another article that alludes to best mpg @ peak torque. Hit ctrl-F and search for peak.
Diesel Economics and Fuel Mileage, TheDieselPage.com

Here is another article
Technology

I did a Goggle search on this subject about 1 1/2 years ago, and it seemed to find alot more information on the subject then it does today. I performed a test like your's this past fall while driving through western Indiana and Illinois on a calm day. My 2006 Silverado 6.6 DuraMax with 2WD and 3.78 rear end obtained best mpg between 50 and 60 MPH.
If you can consistently get 22MPG at 73MPH, then You Da Man!! I'm jealous!


Take care
Dale

[BillTex]

Originally Posted by Sky

Hi Bill,
I swear the "Brake Specific Fuel Consumption" link once worked on this page, but it no longer works for me. Maybe you will have better luck. Here it is.
TheDieselPage.com Feature Articles

Here is another article that alludes to best mpg @ peak torque. Hit ctrl-F and search for peak.
Diesel Economics and Fuel Mileage, TheDieselPage.com

Here is another article
Technology

I did a Goggle search on this subject about 1 1/2 years ago, and it seemed to find alot more information on the subject then it does today. I performed a test like your's this past fall while driving through western Indiana and Illinois on a calm day. My 2006 Silverado 6.6 DuraMax with 2WD and 3.78 rear end obtained best mpg between 50 and 60 MPH.
If you can consistently get 22MPG at 73MPH, then You Da Man!! I'm jealous!


Take care
Dale

Dale, I will take a read at those tonight, but I was running a little under peak torque (could this be said for either side of the RPM curve?) Not sure, as of course there would be diminishing returns due to friction...
I ran out of good traffic conditions before I could try any further testing and I am not patient enough to run slower than 65, although as the cost of fuel rises, so will my search for increased mpg!
I have consistently returned 21 mpg on the highway for 2 years now. I am getting more miles on the truck so maybe it is bumping up further still (17k miles now).
I don't use this truck daily, it only gets out for tow duty or advnture trips (skiing, yaking, etc). But I will continue experimenting next time out.

I think you have 3:73's on your truck also...

All in all I am very pleased with the diesel, as noted I never got this kind of mpg with a gasser.
Thanx for the linx
Bill

[BillTex]

Hey Dale, just read through those links-good stuff. (the first one was dead for me also.)
The other two confirm what I found last weekend; average fuel economy of 20.2 mpg (appears typical according to their reports/data) for the entire trip and my peak efficiency as quoted from Technology "In general, engine fuel consumption depends greatly on the combustion efficiency and often has an inverse relationship with the torque curve. The above figure shows that the fuel consumption is at a minimum (and fuel efficiency at a maximum) at 1,800rpm."
Exactly where I was running at 69 and 73 mph.

Thanx.

Bill

[slowmover]

Originally Posted by Larry C

Our weights are a bit different, because of the 115 gallons of extra fuel in the aux tank...mine will be around
Truck weighs around 8,000.
AS (depending on how it's loaded) around 8,000.
This is the second Dodge/Cummins, and we love it! Especially when in mountains, I equipped this one with the exhaust brake, and I've never felt safer when goiing downhill.
Take care
Larry

My weights are in the signature (CAT Scale). The truck is unloaded for traveling in this number, and the trailer is as ready for travel. Travelling, my CAT Scale tickets are around 15,700#.

An exhaust brake is on my list, probably PacBrake.

Thanks for the reply. I'm pleased to see, in general, that my numbers are in accord with other rigs. Now, with numbers from a CTD pulling an aero trailer I KNOW that I have "good" numbers!

[ebair]

2000 K2500HD 6.0 gas w/ 6500 lb 31 ft AS avg 6.9 mpg

2003 K2500HD LLY Dmax w tuner & 6500 lb 31 ft AS avg 20.5 mpg

2006 K2500HD LBZ Dmax w tuner & 10,000 lb 34 Ft AS avg 14.7 mpg

avg mpg from vacation trips round trip from OH to various points West avg length 4-6 k miles

There is so much difference in the comfort level towing with a diesel I would not consider using a gas engine vehicle again (at the weights I'm towing)
I usually tow at 65 - 70 mph

[Glenn G]

What is LBZ vre. LLY ?

[Sky]

Originally Posted by Glenn G

What is LBZ vre. LLY ?

Hi Glenn, if you are asking differences between LBZ and LBY go here. If you are not asking this, read no more

06 LLY and LBZ Differences - Diesel Place


In a nut shell:
The 2006 Silverado HD and Sierra HD with the Allison 6-speed automatic transmission will be equipped with a significantly revised 310 horsepower DURAMAX 6600 (LLY) through the third quarter of 2005. It will be replaced in these applications in the fourth quarter of 2005, by the higher output, 360-horsepower DURAMAX (LBZ).

[Larry C]

Originally Posted by MrAgent007

My 2006 Hummer H1 Alpha with the Duramax engine and Allison transmission combo with 19,000 miles gets me approximately 20 MPG highway and 17 MPG in city. Towing my 2006 22-0 CCD gets me approximately 15 to 16 MPG highway. Highway speed defined as 55 to 60 with rpm at no greater than 2000. So far, the Duramax/Allison combo in the H1 has resulted in reasonable fuel milage. Hopefully it will improve as the engine gets completely broken in.

Bill,
In regard to the post by MrAgent007 and his fuel mileage on his H-1 Hummer.
I have some contacts at AM General, so I forwarded his post to them, please note their comments below. This comment is from a person who is an automotive engineer, and works with this vehicle every day...
Larry




John W. Camblin

Test Track Administrator

AM General LLC

(574)291-6802 office

(574)291-9594 fax

john.camblin.jr@amgeneral.com



From: Martin, Peter
Sent: Mon 3/17/2008 10:10 AM
To: Camblin, John Jr.
Subject: RE: Hummer fuel mileage

20 highway and 17 city? Thats BS if I have ever heard, unless he has a block so he can only go 1/8th throttle.

0-60 in 7 miles...

[safari 28]

For what it is worth, yesterday on the road ( I have put the 28 in storage till october as my towing season is in the winter) and some how got on the road dog channel on sirius. It is dedicated to the trucking industry. Anyway, very interesting as they update lots of things like new engine and chassis combos etc. Seems Ford and harvester have bad relations and will be done with each other in a few years. Cat may get out of the trucking segment. The big talk is fuel mileage. Seems the 4.00/gallon mark was the break point. They say all the big companies and many independents are reducing speeds to 58-62mph. Claims came in that even with full loads of 50k lbs and more that mileage went from 4 to as high as 7 mpg. One guy went from $1500.00/week to 900.00 just by slowing down. I did notice many were under 70mph. The national companies are asking drivers to check tire pressure once/week and not billy club them. So for those towing at least one good thing will come out of this, semis will not go by at 80+ as much as they used to.

[Glenn G]

Sky,
Thanks for the info ... It was exactly what I wanted.
I don't know if and when I go to diesel but it's good, I think, to keep up with the new automotive tech advances.

[mrmossyone]

Here is the bottlom line for me. If you don't need a diesel it's a bad value vs gas. Someone tried to devalue the point of the extra cost of the diesel by saying no one pays sticker. Well if you don't pay sticker for the diesel you certainly won't for a gasser (actually they are much more likely to give you a bigger break on the gasser, cuz they have got more to get rid of). The cost difference remains, you will pay between 5,000 and 7,000 more for a diesel and you will pay between 60 and 90 cents more per gallon for diesel. At 3.oo per gallon for reg.gas vs 3.60 per gallon for diesel to go 500 miles will cost you 125.999 for gas at 12mpg and it will cost you 112.50 for diesel at 16mpg a diff. of 13.49 per 500 miles. At that rate to make up the cost difference you will have to drive the diesel about 250,000 miles. At 25,000 miles a year that is 10 years of ownership. Very few people will keep their truck that long or for that many miles, the vast majority will never see a savings from diesel. You also will not get much more for your diesel at 250,000 on resale vs. a gasser w/ same mileage maybe 500.00 to a grand more. This does not even take into account how much more a diesel costs to change the oil, regular maintenance and tune ups. Not towing the gasser will edge a little closer to the mpg of the diesel (maybe 1 mpg average w/ in town vs hwy). I would have to look up those maintenance costs to give a precise cost difference but I would say given the extra maintenance costs of diesels you will need to keep the diesel for 300,000 miles to begin seeing a savings.

That being said if you need a diesel none of this matters. However most people towing Airstreams don't need a diesel. I would be willing to bet that you start seeing more people towing with gassers if the price of diesel continues to increase more than the increase for gas. Yesterday in West Memphis, typically where gas is cheapest around these parts gas was 3.09 per and diesel was 3.99 per. At those prices it costs 128.74 to drive 500 miles in the gasser at 12mpg and 124.68 for diesel at 16mpg. That would mean you would have to keep the diesel for about 750,000 miles before you will see any savings over a gasser.

I had the opportunity to speak with a man who works for a company with a large fleet of trucks and he said they have started buying new trucks and they're replacing all their diesels with gassers. He said they ran the numbers and they expect a substantial savings in their truck fleet.
My 3/4 ton hemi would tow up to 10,000 lbs even using the 80% rule that's 8,000 lbs, very few people towing Airstreams are towing more than that. Most people have more truck than they need.

If you have a diesel because you prefer a diesel then none of this applies to you and if you have a diesel because you need a diesel then none of this applies to you. This only applies to people who are thinking of getting a diesel because they think they need one or they think they will save money.

I have spoken, now all of you diesel owners can begin poking holes in my logic and numbers, have fun.

[NevadaGeo]

For people that have a time factor in their pay, there will be a cross over some where that saving 40% in fuel will be overtaken by using 40% more time on the road.
Anybody ever try to drive across Nevada, Utah, Wyoming at 55, late for a job

[BillTex]

Mmo-which diesel have you run in the past that you are making your comparison to? Maybe you had one of the lemon F 6.0l or? Not sure I could not see any of your posts that related to diesel truck ownership…

I don’t really say one is “better” than the other, there are too many variables for each owner (I haven’t owned a diesel long enough yet to make that statement. The long term will tell…) but I have owned, and towed, with both diesel and SMALL block gassers (the disparity would be greater if we compared BB gasser-which I have never owned.) and I can tell you, I am still ahead of the game with diesel. As this post is (was) about fuel mileage of gas vs. diesel, I don’t think anyone could successfully debate that gas is an advantage.
This was my first diesel and I had some reservations, mostly related to maintenance, and fuel availability, but those concerns proved to be unfounded (you might take a look at this thread for a more detailed analysis; http://www.airforums.com/forums/f463/reflections-3-4-ton-ownership-39826.html#post521713). One of the reasons we bought diesel was for economy, and that still holds true.
As far as long term (trade in) I just ran edmunds for my truck (which I also used as a tool when negotiating purchase) and they show a $5200 advantage for diesel right now, so I guess trade in works in diesel’s favor also.

Historically diesel has been less expensive than gas, over the past year this has changed. Maybe after heating season the cost will revert to historical levels. Next year? Who knows…?

I still don’t personally know anyone that has paid, or would pay, a $7000 or $8000 premium to get into a diesel, that’s just nonsense. When we bought our D/A we were making direct comparison’s to gassers. We paid a little more than $3 grand to get the diesel, close enough to sway me. (Maybe too much for others…?)
The factors that were important;

• Range (miles/tank).
• Annual fuel costs

We are still up, even at today’s fuel cost. Am I happy diesel is $4/gal-no way!
Maintenance costs? Actually, I have spent less so far on the diesel, as the intervals are longer, and diesel spark plugs are dirt cheap!

I think folks who are in the market and reading these threads can appreciate hearing the experience of those who have run both gas and diesel None of my stats are second hand, they are our real experience over the last few years.) and can make a valid comparison. (Kind of like Midamrails’s post regarding Chevy vs Ford-what a perspective! ( found here; http://www.airforums.com/forums/f463/08-f250-350-vs-2500-3500-diesel-4x4-crew-cab-40511-2.html).

Oh yeah, we do keep our vehicles for 10 years…maybe I will change my tune then, but so far so good.

Bill

[mrmossyone]

You must be the exception. As far as resale, does your truck have 250,000-300,000 miles on it like my hypothetical situation. You would be lucky to even get 5500.00 dollars for it let alone getting 5500.00 more than a comparable gasser. I'm also sure lots of people would love to know where you got that wonderful deal and I said 5,000-7,000 more not 7,000-8,000 more. As your edmunds book value on your newer diesel shows it would cost over 5,000 more for a diesel over a comparable gasser. I have never owned a diesel because I have never needed a diesel and the numbers have never proven it to be the better value, that doesn't mean that I can't add/subtract/multiply/divide/so on and so on.

Hey you obviously like diesels and obviously prefer them so my post should not even have applied to you. The original poster was interested in wether or not they should get a diesel over a gasser so I believe they would be interested in all cost analysis information not just which gets better gas mileage. Also just for your own information there are gasoline powered vehicles that can tow some Airstreams and get better gas mileage than 3/4 ton diesel trucks do. It's all in what you need and what you want that goes into your own value based decisions.

I'm glad you are on the winning side of value for the product you own but I personally know of 4 people who are now sorry they purchased a diesel over a gasser. All but one are family members and two live right next door.

I disagree that a diesel is a better value if you don't need it, you have a differing opinion and your evidence does not sway me to change mine. That's all. But I won't say your information isn't valid as some people in particular circumstances will surely find your information valuable. I hope you continue enjoying your diesel. If you really want to save some money go used veggie in that diesel.

[safari 28]

I looked up all the quotes I had sitting around from the october lease of our 08 f150. In ALL cases I had them quote a purchase vs lease and from Ford and GM the difference was over 7500.00 diesel vs gas and the lease rates were from 225 to 310/ month more. 2-3 year old loaded f150 lariats have a huge demand. In some cases on leases of 3/4 ton diesels, rates almost doubled. That was the main reason I went with our 150, when leasing and under full warranty, why rent more truck than you need??? The lease rates were higher on 250 series as they have a higher depreciation rate the first 3 years. The expected market is smaller for a 3/4 ton as well and harder to sell. In the end you are paying for the use of the money, either yours or the lease company. Remember, when you have 7500 tied up for 10 years, what can that return on some other income producing investment even for those who buy and have no business deductions. I say diesel is very expensive luxury and ROI sucks for most of us in this forum. Now take in the difference of diesel vs e85, running over 1.30 gallon and a CPA would have to explain why it makes sense to go diesel.

[safari 28]

Originally Posted by NevadaGeo

For people that have a time factor in their pay, there will be a cross over some where that saving 40% in fuel will be overtaken by using 40% more time on the road.
Anybody ever try to drive across Nevada, Utah, Wyoming at 55, late for a job

Recovering from a very long night at the bar. Good friend who owns a fleet of 150 trucks just put the hammer down on anything over 62mph. A truck drivers time is not worth what his truck consumes at 80. They are watching the gps boards and calling the driver if he exceeds the new limit. He does not want to put speed limiters on yet, but will. They cover the west coast from michigan. I feel sorry for the drivers, can you imagine how long that round trip is every week now?

[overlander63]

Originally Posted by safari 28

Recovering from a very long night at the bar. Good friend who owns a fleet of 150 trucks just put the hammer down on anything over 62mph. A truck drivers time is not worth what his truck consumes at 80. They are watching the gps boards and calling the driver if he exceeds the new limit. He does not want to put speed limiters on yet, but will. They cover the west coast from michigan. I feel sorry for the drivers, can you imagine how long that round trip is every week now?

That's 180 miles less per ten hour day. Or, two and a half hours' more driving time at the new limit. That would be an extra day's travel time per 5 day week.

[safari 28]

sucks to be truck driver these days. I think he said before the new rule top speed allowed was 71.