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Diesel Pusher Mileage When Towing

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I have a Cummins ISM (400 hp) with the Allison 3000 MH transmission. I drive at 1650 rpm (that is about 62mph). I consistently get 6.5 mpg. I have done a few things such as change out the muffler to a straight through turbo type and even put a chip on the engine, still 6.5. I have decided this is all I will ever get. You guys that claim higher mpg's consider yourself very lucky!!! I go down the road pulling a Camry on a dolly and just over 38500 lbs GVW.

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Cummins DP 400, Allison 6 speed, 2012 Winnebago Journey 42E. About 7 mph is all we hope for. We're towing a 2012 Cadillac SRX. We didn't buy this beast for fuel economy! We didn't do any better when we had a 1999 Dodge RAM 2500 pulling a 28' Cardinal. We don't whine about fuel economy. I like to say, "If you can't afford the fuel, you should've have spent that kind of money on the vehicle!".

Ouch! "7 miles per hour is all we hope for"

Should get good fuel mileage at that speed! LOL

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We just returned from a trip, our first long trip with our 2002 HR Imperial (40PKD) Cummins ISL 400 Allison 3000 6spd. We were towing our Jeep Wrangler which I believe weights around 3300lbs. We had the generator on for 7hrs of the 11 hour trip North on Rt81 from Tennessee. MPG 10.05. I ran the hills at 55 the flats at 62, I tried to accelerate above 62 down hill for the next hill to get a little extra push and I actually stopped to top off the tank, I thought the fuel gauge was broken. We were very happy with the MPG and impressed with the power. :)

At 60mph I was at 1500RPM's seemed a bit high but MPG was better than I expected, compared to our old coach (1998 Coachmen Santara 311SB Class C V10) this journey would have been 6mpg towing with the engine next to me at 4000 rpm most of the time.

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340-horsepower Cummins® ISB 6.7L turbo-charged diesel engine (brochure), but when
I looked up the engine at the Cummin's site it says its 350HP, confused?
We are having extra's done so not in possession right now, I can't remember
when test driven ... sorry

I have the same engine in my 2008 Winnebago Destination 39W. It came with the Allison 2500 transmission. I asked Cummins when it was in for service if they could upgrade the cuip to 350 hp. Althoug the 6.7 is capable to be computer upgraded to 350 hp, they told me that the 2500 transmission would not be able to handle th additional torque. That's my story and I'm sticking to it.

We are on a trip from near Galveston, accross to the panhandle of Fl, down the coast and back up to Georgia and points North. Except for some time on I-10 we have avoided the Interstate as much as possible. Georgia to Fort Meade, MD was 99.9% on US-17 and other secondary roads, 35-45-55 mph and occasionally 60 mph speed limits. Stop and go traffic is a norm on secondary roads. We are pulling a 2013 Lincoln MKX. I use the built in gauge on the freightliner instrument panel to see what my MPG is. Having compared it to actual calculator methods, the diffference was minimal, about 1/10 mpg of a difference.

On this trip it is sitting on 9.4 mpg. My last two trips out were above 9 mpg. One trip out West bucking head winds and hills it was as low as 7.6 mpg. That was the only trip I ever got below 8.6 mpg.

I drive with the economy mode turned on with few exceptions, like if I notice the transmission shifting frequetly.

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The last 27,000 miles or so we have averaged 8.7 pmg towing a 4 door Jeep Wrangler. Traveling back and forth between Vancouver Island and southern Mexico. 2002 Itasca Horizon with 5.9 Cummins and 6 speen Allison. We NEVER get up to 70 MPH ("Well Hardly Ever" as the Lord of the Admiralty said). Seriously, usually cruise at just over 60 to 65 MPH. We will speed up if traffic is heavy and all lanes trying to go faster. We shift into 6th at about 58 mph on level roads.

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Well just a little update. I am camped on the banks of the little Bighorn river tonight. My last post was leaving Yosemite. I spent about a month in California where the max speed when towing is 55. I decided to drive at 55 or slower just to see if it made a improvement in mileage. As I have posted before I can't shift into 6th till 65 so I never got out of 5th gear in California. I got around 8.8. I since have driven 101 to Washington then back East to Yellowstone and now at the little Bighorn river tonight. after LEAVING Washington my mileage has gone up to 9.829. I have crossed lots of high passes and have tried to cruse at 65-66. I don't get any benefit from running 55 in 5th gear as my time in California proved.

Bill

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I don't really have any info as I drive a v10 gasser and get about 8mpg towing a Saturn. But one interesting thing in this discussion is mpg vs. safety and law. If you are towing in California, the speed limit is 55. In some states, a vehicle as large as the ones you are discussing should obey the 'truck' speed limit. I know that this discussion is about getting the best fuel mileage, but safety and law should have some place in this discussion.

Is it 'safe' to drive your coach at 65 miles per hour, even when the auto traffic is traveling at that speed, in congested city traffic, just to get good mileage? It is unfortunate that manufacturers gear their rigs so that the best mileage is achieved above safe and/or legal speeds. In my rig, I only have a four speed tranny, and it shifts into fourth at about 52 mph, so that gives me lots of room to drive at 55 or so and not have to worry about it shifting down.

One other thing I have noticed, is that if I leave it in 'cruise control', it will shift down much sooner that if I turn it off as I approach a hill, and just use the accelerator to control speed, keeping the mpg higher. Sometimes I long for the 10 speed manual transmissions I had in the trucks I drove so as to get better mileage in my coach. Better control over the shift points. 425 hp, 70,000lbs getting 7-8 mpg. 100,000 lbs getting 6mpg.

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My records show my lowest MPG for a single fill-up to be 5.8 MPG and the highest so far is 12.0. That's quite a variation so I don't worry about either extreme. I have kept track of every mile and every gallon of diesel since we purchased the Windsor, November 14, 2003. The coach had 1235 miles on it when we purchased it and those are not included in these statistics. Having now driven it 127,364.1 miles and used 15,287.2 gallons of diesel my long term fuel mileage is 8.3 MPG.

We were full time in the coach for 7 years and part time (summers) since. I have a Cummins 400 ISL with Allison 6 Speed Transmission. My target highway speed is 62 MPH, just above the 60 MPH speed for 6th gear. Our current toad has a speed limit of 65 miles per hour. I have adopted a fuel saving driving style, sacrificing speed for MPG when going up hill and letting gravity accelerate for me on downhill grades. I drive almost entirely on cruise control using the up and down speed buttons to adjust speed whenever possible. I also minimize brake usage whenever possible by anticipating slow down and stops and coasting while speed decreases. I drive in economy mode when I remember to hit the button.

We have towed two different vehicles 4 wheels down during this period, both were SUV's, Chevrolet Trailblazer EXT and GMC Acadia. Our coach weight is around 36,000 pounds loaded and the toad(s) around 6000. We use our generator frequently while driving in hot weather and also when stopping overnight at Wal-Mart or other non-powered locations. The fuel used by the generator is in the above calculation and currently the usage stands at 1634 hours. I have not compensated for the generator fuel usage, it is in the fuel total above and compensating might increase the fuel mileage slightly.

Our travels have taken us all over the US and Canada including a single trip to Alaska. We have children and grandchildren in Missouri and California and thus regularly make trips across the Rockies and the Sierra Nevada. In good years that is one trip in each direction but there have been exceptions. So there you have it. As I tell my sticks and bricks friends, "It's terrible mileage for a car but not bad for a house. I'll bet that my house gets better fuel mileage than your house!"

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“ But one interesting thing in this discussion is mpg vs. safety and law. If you are towing in California, the speed limit is 55. In some states, a vehicle as large as the ones you are discussing should obey the 'truck' speed limit. I know that this discussion is about getting the best fuel mileage, but safety and law should have some place in this discussion.”

Cookie, perhaps you need to re read my last post and explain to me what I was doing wrong. I was one of the only people following the 55 speed limit and truckers were in the majority in violating the speed limit laws. You can also explain why you think it is safer to be the rolling road block driving 50-55 in a 65 or 75 mph speed zone rather than maintaining the speed and flow of the rest of the traffic.

"Is it 'safe' to drive your coach at 65 miles per hour, even when the auto traffic is traveling at that speed, in congested city traffic, just to get good mileage? It is unfortunate that manufacturers gear their rigs so that the best mileage is achieved above safe and/or legal speeds."

I am wondering where you came up with this gem. I can find nowhere in the preceding post where anyone talked about doing this.

Bill

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My "safe" is the speed limit when logical and the prevailing speed the traffic is moving when congested. We use interstate 80 through the mid west a great deal. In the early morning I can drive 60 mph. However by 10 AM the traffic is running 70 to 80 mph. I follow the normal flow which is likely 70 while the 80 mph cars pass me. If I can find a few governed semi tractor-trailers I will follow them at 62 mph. I do not want to be the road block that gets to fix the rear end. My sweet spot speed is about 62 mph at 1600 rpm towing a Jeep Wrangler. I do not bother with mph calculations because they do not matter - you either go or you stay home - choose!

Glen Burgess

2004 Country Coach - 42' tag

2013 Jeep Wrangler Unlimited

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After 10,000 miles towing our 5400# Jeep Commander, we are averaging 5.5 MPG. Speeds from 58-72 MPH, with maybe 10% in mountainous terrain. Engine is CAT C15 ACERT with 600 BHP and 1950 ft lb torque.

American Heritage 45E

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Maybe you 'Gurus' out there can answer a question for me. I drove truck for 40 years. The last few years I drove either an International with a 460 hp Cummins and an 8 speed Eaton automatic transmission or a Volvo with a 470 hp Volvo engine with a 10 speed Volvo automatic transmission.

I drove all over Washington and Oregon, east and west parts of the states, going over the Cascade Mountains often. I would drive about 80,000 miles a year. I would have weights anywhere from 42,000 to 101,000 pounds, averaging about 65,000. I would drive the 'truck speed limit', as demanded by my company, enforced by a very accurate GPS based computer system in the truck that they could monitor. I would average about 8.1 mpg for the year. Also, the vast majority of those miles were hauling two trailers, with a total of 7 axles (26 tires) on the ground, which, of course adds tremendously to the rolling resistance, thus decreasing fuel efficiency, not to mention the huge amount of wind resistance caused by all the wind blowing around between the trailers. Also, the tractors were 'dual drive', meaning that the power to the road went through two drive axles, causing another loss of fuel efficiency.

My question for a long time has been "Why do Motorhomes get such lousy mileage?" Our tractors by themselves weighted about 25,000 lbs, and we could get 11-12 mpg 'bobtail' (not hauling a trailer). If we added the trailers, empty, another18,000 pounds, it would drop to about 9.

With a motorhome with only about 45,000 lbs and three axles with a total of 8 tires on the ground, mentioned above getting 6.5 mpg, my question is "Why".

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I think the biggest reason is gears. You only have 6 in the motorhome but up to 10 in the truck. You have a much better chance to be in the best gear for a given situation.

Bill

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Cookie,

While weight is a significant factor in determining MPG on grades, it is far less important than aerodynamic drag when running on flat ground.

While there are many factors affecting MPG, there are a couple that reduce motorhome MPG as compared with trucks. While trucks have their cooling systems (Charge Air Coolers and radiators) in front where the vast majority of the air flow is provided “for free”, diesel pushers have to run fans that provide this cooling—often at a cost of over 25 HP.

Modern trucks are more aerodynamic than motorhomes. There is nothing “slick” about the aerodynamics of a motorhome, with its multiple awnings and multiple roof top items.

And, with 10+ gears, trucks can operate closer to peak torque RPM which is more efficient than at higher RPM.

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I wonder what the manufacturers could do if they had the incentive. The engine makers don't seem to feel any pressure to improve the mileage in the motorhome industry as it isn't that big of a market compared to the over the road/commercial market. The only advance in engines is the new Cummins V8. I don't think it would be that hard for Allison to upgrade to at least a 10 speed. The makers could do a lot to streamline the coaches and get better airflow but there isn't anyone demanding that they do it. Well not anyone they are going to listen to.

Bill

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I've been looking at motor homes on the highway and I see lots of changes. They are small changes, but if you look at the newer coaches, you will begin to see some significant differences. Not all manufacturers are making changes but some have made their coaches more aerodynamic. Some of the changes I've seen is that many have eliminated or minimized the number of awnings. There is a trend to tinted windows instead of awnings. Window fittings on many coaches are smooth on the outside of the coach, no indentation and if they do open the opening glass is very closely spaced to the fixed glass. When coaches have awnings, which I like for their effectiveness in eliminating heat from the coach, the awning support arms are much smaller and more streamlined and the awning roll is smaller and more sleek. Slides on many coaches fit much more seamlessly into the sides of the coach than on older coaches. I haven't noticed significant changes in the aerodynamics of rooftop features but that may be happening, just a little harder to observe. I know there are coaches without rooftop air conditioners.

We know that more modern engines are designed for less pollution but they are also more efficient with electronic fuel injection and other design features that improve fuel mileage. We will always be a consumer of the over the road power train components because as Bill said, we are not a very large market. In fact I suspect the size of our market is becoming smaller, not larger. I believe that all manufacturers have a strong incentive to make their vehicles more efficient. As fuel prices continue to rise, many buyers are factoring fuel efficiency into their purchase decisions.

Of course one of the major factors affecting fuel mileage is the operator and that is beyond the control of the manufacturers and designers. They have given us tools to improve our mileage, it is up to us to learn to use them the best way possible to improve fuel mileage. As you may have noticed, many trucking companies have their truck regulated so they can't exceed a set speed. Others monitor their drivers with GPS equipment to assist with route planning and to monitor speed compliance with company policy. UPS has their GPS units set for only left right turns where possible. Are we ready to accept any of those changes? This change is in our hands, all we have to do is follow accepted industry practices for increasing fuel mileage.

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Brett and Tom well put. I always enjoy reading your responses.

Tom, what MPG are you averaging? My coach is similar just two years older non EGR engine) I assume yours has EGR?

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Wolfe10

Thanks for the input. Your info does make sense. I guess one question then, is, why not put more gears in the motorhome? Allison does make 10 speed trannys for trucks, why not put them in MHs? I forget the manufacturer, but there is one that does install a 10 speed in Motorhomes. Another thing I just thought of is that the lighter the weight, (45,000 compared to 101,000, or even 80,000) the less important the number of gears is. That's why one can get along with four gears in a car, but not so well in a Motorhome with four. While weight may not make much difference on the flat, there is a LOT of difference between 45,000 and 101,000 pounds (triples, three trailers hauled by one tractor) not to mention the additional number of tires on the ground. The aerodynamics are, of course a big part, but the difference between 8.1 and the 5.5-6 that I read about in these forums seems a bit much. Except for the aerodynamics and the engine in the rear (which I had not even thought about), it seems that there could be some changes made that wouldn't cost much money, especially as a percentage of $300,000.

After driving a diesel for 3 million miles, and looking at the mileage I would drive my Motorhome (5,000 miles per year), I figured that the additional outlay for a diesel was not worth it. So, I plug along in my gas rig getting 8 mpg and not having any problem getting fuel. To each his own, but, in my case, didn't need to have a diesel just for the sake of saying I had a diesel. Been there, done that. :-)

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Wolfe10

Thanks for the input. Your info does make sense. I guess one question then, is, why not put more gears in the motorhome? Allison does make 10 speed trannys for trucks, why not put them in MHs? I forget the manufacturer, but there is one that does install a 10 speed in Motorhomes. Another thing I just thought of is that the lighter the weight, (45,000 compared to 101,000, or even 80,000) the less important the number of gears is. That's why one can get along with four gears in a car, but not so well in a Motorhome with four. While weight may not make much difference on the flat, there is a LOT of difference between 45,000 and 101,000 pounds (triples, three trailers hauled by one tractor) not to mention the additional number of tires on the ground. The aerodynamics are, of course a big part, but the difference between 8.1 and the 5.5-6 that I read about in these forums seems a bit much. Except for the aerodynamics and the engine in the rear (which I had not even thought about), it seems that there could be some changes made that wouldn't cost much money, especially as a percentage of $300,000.

After driving a diesel for 3 million miles, and looking at the mileage I would drive my Motorhome (5,000 miles per year), I figured that the additional outlay for a diesel was not worth it. So, I plug along in my gas rig getting 8 mpg and not having any problem getting fuel. To each his own, but, in my case, didn't need to have a diesel just for the sake of saying I had a diesel. Been there, done that. :-)

I too have several million diesel miles under my belt. Now retired, I drive my RV 10-20k per year. The reason they don't generally put 10-13 speeds in RV's is because it is not necessary. All transmissions have final ratio in the neighborhood of .87 and if gross weight of vehicle and power/ torque of engine allow, the least number of gears possible to obtain reasonable performance is preferred. Heavy haulers require more gears to narrow engine performance and keep engine in sweet spot thru entire operating range, but these trucks may gross over 100k.

I have operated motor homes with high cube V8's, & V10's and there is no comparison between them and a well designed diesel. My puny little Cummins 275 is blast to drive compared to any gas unit on long trip. If spend significant time on road, diesel is the only way to go.

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I think it is real interesting that we spend 500,000 dollars on a Motorhome and then try to figure out how to get about one half more mile per gallon out of it......Has anyone really thought thru the diffreance in 7.5 miles per gallon and 8 miles per gallon in the grand scheme of things.

The same thing goes for if I tow or not, we always wonder how much it cost us to tow the car, that we want to have with us (the cost of which is about $40,000) to go running around burning gas to see what we could not see from the windshield of the half a million dollar MH...

We are really funny humans I recon..........If you get 7.5 Miles Per Gal at 4 bucks per gal the saving will be about 27 cents for that 1/2 mile. I must be missing something here.....

I buy fuel at a Pliot/FlyngJ so I can save about 2 or 3 bucks on a fill up. I guess a penny saved is a penny earned but I will just go ahead a drag that 24" enclosed trailer around and waste that .27 cents per gallon cause I like to have my car with me....I'm done.........

Mike

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I am getting 7 to 7.5 miles to the gallon of fuel. I was towing a 27 ft I closed trailer with a motorcycle and a car total weight was about 8000 lbs. I traded in the trailer for a tandom tow dolly total weight is now 5000 lbs. my mileage has improved to about 8 mpg. My motorhome is a 2014 Winnebago Journey 42ft w/tag axle. I think that my mileage is pretty good. My tach is at 1600 in 6th gear.

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