Well, I know this is not the first, and wont be the last post, but I haven't found what I am looking for yet. I have read until my eyes are crossed, but not getting any real answers. I just drove down to pick up our new camper, a 30 foot toy hauler. Empty weight is about 8100# and I probably had it at 9000# with water and a few minimal things like beer and food:) I averaged about 8.5 MPG. The best I could see was 10+ on the flats with a tail wind. I even babied it at no more than 65 for 2 tanks just seeing how good I could do, still averaged 8.7. That seems WAY low to me. I know, you ask what are the details:

- stock truck with 35k miles(wheels and tires in garage are back to stock)
- running between 65 and 70 mph
- no strong head winds or tailwinds
- Hand calculated and DIC, both are within 1 tenth of each other

So what is going wrong? It doesn't seem right. Is there anything I can do?

DPF Delete! EFI Live! Will improve things alot, however that is a big trailer. Is it a fifth wheel? If it is, that will be alot of wind resistance and hard to improve much on. If it's not then you should do slightly better. Don't expect much better than 11-12 at best with a trailer that size..

DPF Delete! EFI Live! Will improve things alot, however that is a big trailer. Is it a fifth wheel? If it is, that will be alot of wind resistance and hard to improve much on. If it's not then you should do slightly better. Don't expect much better than 11-12 at best with a trailer that size..

Its a bumper pull, but its tall though. Just seems like there are folks out there doing a lot better. A friend here has a 30 foot fiver and gets 15 with an 03. Granted its an 03, but almost twice as much MPGs?

You left out one of the most important details, what terrain are you pulling across? You give a strong hint that you get 10+ on flat ground, probably means you have hilly terrain because that's the "norm" that most folks get unless they are pulling short campers in the mid-west.

I get between 7 and 8 on I-79 and I-68 (WV and Maryland) and 10-11 on I-80 and I-90 (flat lands through Ohio and Idiana).

Also you didn't mention whether this was on winter fuel or summer fuel. I get 1 mpg worse on winter unloaded and 2 worse loaded.

Also didn't mention how long your trip was. If it was short the warm up time really eats into your mileage. I drive 32 miles every day, I burn 1 gallon in the first 10 miles and .8 gallons over the next 22......just something to think about.

On hilly terrain, winter fuel and a short pull I've gotten 6-7 mpg, on a long pull in the summer across Ohio I've gotten 12.

You left out one of the most important details, what terrain are you pulling across? You give a strong hint that you get 10+ on flat ground, probably means you have hilly terrain because that's the "norm" that most folks get unless they are pulling short campers in the mid-west.

I get between 7 and 8 on I-79 and I-68 (WV and Maryland) and 10-11 on I-80 and I-90 (flat lands through Ohio and Idiana).

Also you didn't mention whether this was on winter fuel or summer fuel. I get 1 mpg worse on winter unloaded and 2 worse loaded.

Also didn't mention how long your trip was. If it was short the warm up time really eats into your mileage. I drive 32 miles every day, I burn 1 gallon in the first 10 miles and .8 gallons over the next 22......just something to think about.

On hilly terrain, winter fuel and a short pull I've gotten 6-6 mpg, on a long pull in the summer across Ohio I've gotten 12.

The trip was 800 miles coming from Winslow, AZ to Colorado Springs, CO. I wouldn't consider that stretch of road hilly, as I am facing much hillier terrain in the Colorado mountains. I should clarify that when I was seeing 10-11 on flats that was more or less "instant" economy by resetting the DIC and giving it 40 or 50 miles. It didn't seem to matter much what I was doing, speed, hills, wind, it all averaged to 8.5 MPGs.

Tuning would be the only way to possible increase mpg. But here you can void your warranty altering the ECM programming. Some Duramaxes do great others do not. Who knows why. You can have two exact same setups and run side by side and one may do way better than the other. For my experience babying it seems to just make things worst. These trucks need to work and work hard to get better mpg. For some it may be over 100,000 before they see improvements.

Brian

That is part of having a big truck hauling a big load. I get the same millage as you towing my 5th wheel with my dully. You have to remember in addition to any hills you are traveling on you also have a lot of wind resistance and rolling resistance from the extra tires on the road. I have been fighting the same battle with exhaust, programmers, and anything else I could buy hoping to get better millage. My results I managed to improve from 8 or 9 to 9 or 10 after spending over a grand. I would not consider my mods to be of value in the millage department but overall drive ability and the fun to drive factor is well worth my investment. I have just decided to suck it up and deal with the low millage numbers. Because I am not giving up my toys :)

I can get 11-12 towing my 15k boat down to Fl, pretty flat, but i always use tow haul mode and keep it in 5th gear and set my cruise at 62-65 or 1800 rpms, if i go faster by 5mph i will drop 2mpg. And if i let it shift into 6th i loose milage because its lugging and working too much at least thats how i get my best milage.

sounds about right to me I had an 08 worst was 5mpg very loaded and 80mph best 18mpg empty 65mph I averaged 14-16 everyday driving. Remember theses trucks are very heavy the older one were considerably lighter.

Well at least I have comfort knowing some folks are with me. Seems like I was reading several posts from other DP members with LMMs saying they were getting upwards of 14 MPGs pulling big loads. I do love this truck, but its damn expensive. If I had known that I could have paid 1/2 as much for an older Duramax and got 50-75% more gas mileage, I am not sure I would have made the choice I did. Too late now. Would lose my a$$ on this truck just to get an older truck with better gas mileage.

- running between 65 and 70 mph


That's a big part of your problem. Drop to 60MPH (55 if you can hack it, or at least somewhere in between) and watch your fuel mileage skyrocket.

With my 6.5 pulling a 12K fiver I can get nearly 12 MPG (11.7 to be exact) pulling at 90KPH/55 MPH. However, when I step it up to 62 MPH and I'm in the 9.5 range. I typically pull at about 98KPH (~60MPH) and can get 10.5MPG consistently.

To get mileage when towing speed plays a dramatic factor, much more so then when you're empty.

I'd also venture to suggest that your trailer weighs a lot more then you think it does - most 30' trailers are in the range of 9000# empty BEFORE you put anything in them, and one must also remember that the stated empty weight on RV's doesn't include any options - AC(s) for example, full water/holding tanks, and even the empty propane cylinders, much less full ones.

water/meth certainly couldn't hurt things. ask lotsofmiles

That's a big part of your problem. Drop to 60MPH (55 if you can hack it, or at least somewhere in between) and watch your fuel mileage skyrocket.

With my 6.5 pulling a 12K fiver I can get nearly 12 MPG (11.7 to be exact) pulling at 90KPH/55 MPH. However, when I step it up to 62 MPH and I'm in the 9.5 range. I typically pull at about 98KPH (~60MPH) and can get 10.5MPG consistently.

To get mileage when towing speed plays a dramatic factor, much more so then when you're empty.

I'd also venture to suggest that your trailer weighs a lot more then you think it does - most 30' trailers are in the range of 9000# empty BEFORE you put anything in them, and one must also remember that the stated empty weight on RV's doesn't include any options - AC(s) for example, full water/holding tanks, and even the empty propane cylinders, much less full ones.

I agree I think my 26' toyhauler is 10,000empty

I don't believe the engine or exhaust is to blame. It's how heavy the cars and trucks are these days with all the safety equipment. We all know the metal isn't very heavy.

Your problem is that you're moving a whole bunch of weight. That takes energy. Energy comes from burning diesel. The faster you go the more energy it takes. The more diesel you'll burn.

You've also got a tall unit. That means lots of "parasite drag." Below about 35mph that doesn't mean much. By the time you're in the '60s it means a whole lot. It takes energy to "push air."

I pull a 4 horse, LQ trailer. At 12,000 lbs. I get just over 10 mpg at 63-65 mph (hand calculated). My form factor is a lot lower, though, and I'm not pushing near as much air. It's also a gooseneck and that may help some, too.

The hard truth is that if you want fuel economy don't buy a big truck and pull a big trailer. It's not going to work. Or slow down. The next time you go out try running at 60 for a couple of tanks and then at 65 or the same amount of fuel. I'll bet money you'll be surprised by the numbers. :)

Weight, gear selection and speed have been discussed.....and you control all that. I'd suggest run the engine at 1900 - 2300 RPM at any given ground speed. Drive by the tach, not the speedo.

An '08, I believe has a diesel particulate filter in the exhaust. And a re-generation system to keep it clean. I think that will cost you at least 10% more fuel use. Likely still on warranty so think a bit about deleting the DPF. Blame the EPA on that one, more fuel, cleaner exhaust, ha ha.

Are you running a dually? Most better mileage figures come from the single rear wheel guys. Older units had less polution equipment on them and burned less fuel.

34,000 miles... is that driving light, no trailer? Work the hell out of it (with a trailer on the hook) for 5,000 more miles and see if your mileage comes up.

I don't post my fuel mileage because I have had both ends of the spectrum, depending on more variables than I have room to list.

I hate to beat it to death, but privatepilot is right. You need to drop about 10mph off your towing speed.

I can get 11-12 towing my 15k boat down to Fl, pretty flat, but i always use tow haul mode and keep it in 5th gear and set my cruise at 62-65 or 1800 rpms, if i go faster by 5mph i will drop 2mpg. And if i let it shift into 6th i loose milage because its lugging and working too much at least thats how i get my best milage.

x2

I expect with OP "babying it" he was lugging.
I have found holding 5th in T/H up to 74-6 is more efficient relatively than letting it go to 6th.
I tow about 68 w my 8-9k boat and see 11.5-13.5 trip avg.

Its a bumper pull, but its tall though. Just seems like there are folks out there doing a lot better. A friend here has a 30 foot fiver and gets 15 with an 03. Granted its an 03, but almost twice as much MPGs?

Have you actually driven with him and hand calculated your self? Fifteen is hard for me to believe. Best I've seen was 11 and that was down hill from Wyoming to Salt Lake, I avg. around 9 towing as do my buddies chevs which range from 05' to 09'.

Have you actually driven with him and hand calculated your self? Fifteen is hard for me to believe. Best I've seen was 11 and that was down hill from Wyoming to Salt Lake, I avg. around 9 towing as do my buddies chevs which range from 05' to 09'.

Yep, I really doubt he's actually getting 15 MPG. If he's blindly trusting his fuel econonomy guess-o-meter, it's not reading accurately - I know a LOT of people with big (and small) fifth wheels, and 15MPG over any sort of distance is a pipe dream.

Run a tank or three while towing and hand calculate his mileage. I think you're friend will be rudely surprised. ;)

And there's the bigger issue. Some people tend to be a little liberal with their mpg calculations. 15 mpg towing a 30 ft trailer is highly unusual, I'll just put it that way. My neighbor has an '04 Duramax w/ exhaust, Edge Juice, etc, and he doesn't get any better mileage than my LBZ does when we're both towing similar trailers -- travel trailers about 30 ft in length overall.

To quote Dr. House - everybody lies.

There are plenty of trolls on the internet that will tell you that they are getting 20 mpg unloaded in their stock 08 CC/ Dually, and 15 towing their 40' 5th wheel, just to watch you pull your hair out trying to figure out what's wrong.

I get about 8 with the 5er (15K scaled), 11 mpg with the boat (5k ~ ish) and 15 empty mixed city and highway. Add a slight headwind or hills and it can drop to 6mpg with the 5th wheel.

Reduce resistance at every opportunity. Inflate tires to the max, keep your bearings greased, make sure your brakes don't drag, and drive as slowly as you possibly can stand while towing.

Aerodynamic tricks might help too - small tires and no lift, tonneau covers for bumper pulls. Lower the hitch down as much as you can safely both with a bumper pull or a 5er - this will get as much of the trailer out of the wind as possible. If you have a 5er, make sure the trailer is as close to the cab as it can be safely, and get a ventilated tailgate to reduce the wind tunnel in the bed.

Air resistance increases exponentially with speed, so it takes 9 times as much HP to pull at 80 as it does at 40 with all other factors being equal. You can imagine what that does to mileage.

Good luck and post your tips.

I've been on here a while, and don't remember many posts from guys pulling big loads getting much over 12mpg.
Personally 12 is good for me 10 is bad while towing with my LMM. I'm close to the same weight as you, but mine is a boat and pop up trailer so the wind resistance is better on my setup.
I tow in the mtn's every trip, biggest factors to me are driving speed and fighting a headwind. (I consider hill climbing a constant, because it is for me)

Sounds about right I tow a 30ft TT. Make sure you check the tire pressure on your trailer, it does makes a difference in your in your mpg.

Thanks guys! Seems as if this post has brought out some truth to pulling big loads with big trucks. I guess I just need to suck it up. Still cracks me up that folks are claiming to average 14 and 15 MPGs pulling over 10k travel trailers. Maybe they just got lucky. In the mean time I will just go enjoy my toys! In the future I may look to misplace my DPF and add a tuner, but time will tell.

I am a full time RVer and put my cruise @ 60-62mph and get between 10-12 mpg,depending on wind, hills, and temp. I'm towing a 18 k 5ver that stands 13' 2" at the high point ( front ac unit). I have a Hypertech Max Energy program installed and keep 75psi in front tires and 70 in rear duals. Also I make sure my 5ver tires are at the 125psi that the factory calls for. I keep track of all my tires with my Pressure Pro monitor in the cab. I put proper air pressure at the top for good mpg and safety.

Good Luck
orkirk

I'm full time rv ing too and best I've seen is 11 @ 55 on a calm day. Mostly I get 9ish around 65. 70 hurts but I run it sometimes.

guess I just need to suck it up. Still cracks me up that folks are claiming to average 14 and 15 MPGs pulling over 10k travel trailers.\

They're probably not resetting their trip-meter average reading and then towing their trailers for a "Huge 30 mile trip" and then reporting their average fuel mileage again from the trip meter. Of course, it won't have changed much as it could have been averaged over thousands of miles at that point.

Most people with travel trailers tow less then 1000 miles per year, many less then 500. Combine that with someone who has no idea how to properly calculate fuel mileage to begin with, add in a healthy dose of stretching the truth, and you get "I Get 15 MPG towing my trailer!".

My 04 LB7 got 12.3 mpg hand calculated towing my Jayco 34' TT which weighs about 8K-8500 lbs. I left MD and towed it into the hills of NW Pa, then turned south and hit Branson Mo, then west across Texas and up the Rocky mountains to Flagstaff Ar, then back down and into southern Calif to San Diego. Then up the coast of Calif through LA and into San Fran, and then headed east across the plains states into Virginia, and then back to Maryland. I put almost 8K miles on the truck, including climbing the Grapevine in Calif and Eisenhower pass at 13K feet in Colorado and flats of Kansas and hills of the ozarks. I used cruise most of the time, had a camper shell on the truck, had 80 lbs air in the tires, AMSOIL synthetic oil in the engine, and synthetic oil in the differential and Allison. I hardly ever put it into Tow/Haul mode except when it would shift down repeatedly. I always used it going down steep hills. I kept track of my gallons used and miles driven, and when all was done, I only got 12.3 mpg and I was also disappointed because when not towing, I get around 20 empty on cruise on long trips. My DIC said I was getting much better than 12.3, but I don't remember the number. So that's my story. Stock truck still, original injectors still but now out of warranty if they do go.

Why would you not put it in Tow/haul?

I'm full time rv ing too and best I've seen is 11 @ 55 on a calm day. Mostly I get 9ish around 65. 70 hurts but I run it sometimes.


probably what hurts me most. I'm never below 65 unless weather dictates it :D still get a solid 9 with my total weight around 22,500-23,000

probably what hurts me most. I'm never below 65 unless weather dictates it :D still get a solid 9 with my total weight around 22,500-23,000
You have a 6 speed, I don't.

Why would you not put it in Tow/haul?

As everyone knows, putting it into tow/haul causes it to run at a higher RPM which reduces my MPG, and if the truck is not lugging, it doesn't need to be in tow/haul. If it downshifts on its own while climbing a long grade, I will put it into tow/haul mode to keep it from up-shifting.

Secondly, if my truck is able to pull the load without downshifting, and the temps are ok, why put it into T/H mode? I found that if the hill is gradual like most big interstate ones, the truck maintains the load fine. If the hills are steep but short, like those in West Virginia or PA, where you are coasting down a small hill only to go back up a steep grade, put it into tow/haul. It's the speed up, coast down that makes it necessary to put into tow haul.

Another thing I have noticed after pulling my trailers many, many miles, is that if my foot is on the throttle (ie cruise not activated), the truck downshifts more because I don't give it enough throttle as I approach a hill. If cruise in activated, I can feel the truck pull and as long as the path ahead is clear, I leave it in cruise. If it downshifts, I take it out of cruise, and put into tow/haul mode.

One more thing, I have ridden with a lot of people who pulsate the gas pedal too much. It drives me crazy to ride with people like that, gas, let up, gas, let up. I suspect most people don't even know they are doing it.

My total weight of my truck and trailer as measured on a certified scale was 16,710 lbs for a tow behind, so my wind surface is much less than a 5th wheel.

I got around 10-11mpg from FLA to PA up 77 and back 95 towing a 13k 5vr. I towed around 65-70mph and stayed behind 18 wheelers as much as possible. If you let them do all the work moving the air you'll get much better mpg.

probably what hurts me most. I'm never below 65 unless weather dictates it :D still get a solid 9 with my total weight around 22,500-23,000

Have you weighed that thing yet?

I have a 13,000 lbs. 5th wheel I've towed about 40,000 miles so far. Quick starts and a heavy foot while going up steep hills will kill your m.p.g. I use tow haul but I trick it so it lets the truck shift into 5th gear quicker. When my 2001 Chevy is in 4th and 5th gear the torqe converter is locked up even when its not in tow haul. I shut off the tow haul when I get to about 50 m.p.h. and the truck will shift into 5th gear quicker. I turn the tow haul back on when it shifts into 5th gear. This keeps the r.p.m's down and the m.p.g. up. I have an Edge Juice in my truck so I can see what gear the truck is in at all times. I get about 10-11 m.p.g. on average towing. I get 12.5 m.p.g. down south on the flats.

As everyone knows, putting it into tow/haul causes it to run at a higher RPM which reduces my MPG, and if the truck is not lugging, it doesn't need to be in tow/haul. If it downshifts on its own while climbing a long grade, I will put it into tow/haul mode to keep it from up-shifting.

Secondly, if my truck is able to pull the load without downshifting, and the temps are ok, why put it into T/H mode? I found that if the hill is gradual like most big interstate ones, the truck maintains the load fine. If the hills are steep but short, like those in West Virginia or PA, where you are coasting down a small hill only to go back up a steep grade, put it into tow/haul. It's the speed up, coast down that makes it necessary to put into tow haul.

Another thing I have noticed after pulling my trailers many, many miles, is that if my foot is on the throttle (ie cruise not activated), the truck downshifts more because I don't give it enough throttle as I approach a hill. If cruise in activated, I can feel the truck pull and as long as the path ahead is clear, I leave it in cruise. If it downshifts, I take it out of cruise, and put into tow/haul mode.

One more thing, I have ridden with a lot of people who pulsate the gas pedal too much. It drives me crazy to ride with people like that, gas, let up, gas, let up. I suspect most people don't even know they are doing it.

My total weight of my truck and trailer as measured on a certified scale was 16,710 lbs for a tow behind, so my wind surface is much less than a 5th wheel.

Everyone has there own of towing I guess. I will leave the tow/haul on with my 20,000lbs. I would really like to get a cat scale to see what truck and trailer are together.

Just to throw some facts out there, in tow/haul mode the converter locks in 2nd gear and stays locked. In "normal" mode it usually won't lock untill 5 gear or heavy throttle. I've noticed that if i forget to engage t/h while pulling my trans temp runs 20 to 30 degrees hotter, not too hot but hotter.

Take it for what its worth....

Just to throw some facts out there, in tow/haul mode the converter locks in 2nd gear and stays locked. In "normal" mode it usually won't lock untill 5 gear or heavy throttle. I've noticed that if i forget to engage t/h while pulling my trans temp runs 20 to 30 degrees hotter, not too hot but hotter.

Take it for what its worth....

Thats one thing I am really worried about with this truck the tranny runs around 130 right now in 35 degree weather. So what it gonna do in 90 degree weather with 13000lbs behind it?

Thats one thing I am really worried about with this truck the tranny runs around 130 right now in 35 degree weather. So what it gonna do in 90 degree weather with 13000lbs behind it?

Not abnormal, transmissions heat up even unloaded.

Until it gets north of 220 you don't even have to worry, only at ~230 do you have to be concerned - damage starts at 240.

I don't you'll ever see it get anywhere near those temps With only 13K behind you.

Not abnormal, transmissions heat up even unloaded.

Until it gets north of 220 you don't even have to worry, only at ~230 do you have to be concerned - damage starts at 240.

I don't you'll ever see it get anywhere near those temps With only 13K behind you.

I hope your right but I am having my doubts. I don't know alot about the allison but on my old ford they would never even get to 100 degrees in the winter and around 180 to 200 towing in the summer.

90 degrees pulling 15K lb's up 11% grade.....only got to 190 and the engine fan was kicking on quite a bit. You'll be fine, transmission temps are never an issue on these trucks.

The only reason I brought up increased transmission temps in the first place is that heat has to come from some where, and that some where is fuel. If you spend fuel stalling up the torque converter instead of propelling you down the road that extra energy shows up as heat in the trans.

I hope your right but I am having my doubts. I don't know alot about the allison but on my old ford they would never even get to 100 degrees in the winter and around 180 to 200 towing in the summer.

I pull a 12K high profile 5th wheel with my 97 6.5 with a 4L80E (a much less stout transmission than any Allison) and have only seen 220 once and that was a long sustained grade where I kept my foot into it despite being down in 2nd gear out of lockup. 90% of my time towing it hovers around 180 or so, 200 or 210 on grades, only to drop quickly once back in 4th lockup.

I Started out with winter fuel getting 10mpg pulling a 27ft 8k+ bumper pull & Later got great fuel in Las Vegas NV & was getting 15/16mpg @ 68mph for 700 miles.