Improving Fuel Mileage

[tbutler]

Our motor home has a Cummins 400 ISL with Allison six speed transmission. We have a gross weight of 36,000 pounds and our tow vehicle is just under 6000 pounds. And, yes we could travel in a smaller, lighter vehicle and tow. Given that this is our rig, what can I do to reduce my fuel usage? No, I'm not going to park it!

Last year as I was driving through Ontario I was driving to obtain the highest efficiency. I could improve the mileage from 8 to 9 miles per gallon (12%) by using a few techniques outlined below. The press mentioned a number of these techniques called hypermiling. You can check the Internet for entries under the category of hypermiling to find all the suggestions. These are applicable to motor homes.

Simple ideas like finding the shortest route between two points can reduce fuel used without affecting the mileage of the vehicle. Removing articles not needed or reducing levels of water or propane in a storage tank if they aren't needed can help.

Look far ahead to see what traffic changes are coming up. As soon as I see a sign indicating reduced speed ahead I'll drop to a coast. I will even begin to slow if there are signs coming up indicating a community ahead, a water tower or a grain elevator, etc. If you practice this you will find that you are able to accommodate the reduced speed without using brakes. Using brakes indicates wasted energy because you had more energy than needed to get to the point at the speed desired. It is impossible to not use brakes but try to anticipate slowing traffic and speed limits so you don't waste fuel maintaining speed only to find you need to use the brakes to get rid of some of the speed.

Monitoring my turbocharger pressure and keeping the turbo at or below 60% if possible seemed to make the biggest difference. That meant letting the speed bleed off as I climbed hills, topping the hill at a slow speed and letting the motor home accelerate downhill mostly from gravity. This works well on small hills in light traffic. For mountain grades the momentum of the coach won't really do much after the initial climb. Still we didn't shy away from the hills on the North side of Lake Superior as was advised by a truck driver we met near Nipissing. He recommended taking Highway 11, further north across flat country. We of course wanted to see the best scenery, not necessarily make good time so we stayed with Highway 17 and enjoyed the trip. This does slow down our travel but frequent passing lanes assist other drivers in getting past us. I seldom held up another vehicle for more than a mile if that.

Another technique that helps with fuel mileage is to observe traffic signals carefully. If a light is green in the distance it will likely change to red before you get to it. Slow down and let your speed deteriorate well in advance of the light. If this works properly the light will change to green again before you have to come to a complete stop. Then you can continue through without having to come to a complete stop and accelerate from a stop. This process is what causes fuel mileage to drop so much in city driving. I have noticed that truck drivers are using this method. The one drawback here is that some auto drivers will pass you and then pull in front of you, shortening the distance you have to coast before stopping. Auto drivers can also be helpful when a light has a pavement sensor to trigger the green light. A car pulling up to the stop line will trigger the sensor and cause the light to change to green earlier.

Simply observing speed limits will help reduce fuel usage. After all, we are usually the out-of-towners. I make it a point to observe speed limits as closely as possible. Slowing down will improve your fuel mileage. Even though the shift point on my transmission is 60 to go into overdrive, I still get better fuel mileage at 55 or even 50. If the wind is blowing in the opposite direction of your travel (in your face) it is even more important to slow down. Driving 55 miles per hour in a 20 mph headwind is almost the same as driving 75 miles per hour in a no wind condition.

Minimize idling time to reduce fuel consumption. Diesel engines can be shut down after a brief period to cool the turbocharger. If you have been traveling slowly before stopping, no cool down is necessary. Attend a workshop by the engine manufacturers at a convention and you will learn more about this. Your fuel mileage while idling is exactly zero. Avoiding urban areas during rush hour when possible will also help improve your fuel mileage while you preserve your sanity!

Our motor home manufacturer (Monaco) indicates that using the generator to run the house air conditioner is more energy effective than using the dash air conditioner. This may not be possible or correct information for all vehicles but if you have a large motor home with a generator, it likely is true. A generator will run on about a half gallon of diesel an hour. Our motor home will use about 6 gallons of fuel an hour when driving cross country. If you have an instant mileage readout on your motor home you can drive for a while with the dash air conditioner on and then turn it off to see how it affects the mileage you are getting. It does make a significant difference.

Lets start a discussion, please add your own techniques as replies.

[PIPEWRENCHGRIP]

I use many of the same technicquess you use. The shift point on my New Monaco is at 55 so I can travel 55 up to 60 and seem to get about the same milleage. Also I set the transmission in Econony mode, this eliminates nuisance down shifting on small hills but will allow downshiffting on larger hills. You would't use this mode in mountain driving! Also depending on your travel plans, check the weather, which way will the wind be blowing, you might be able adjust your travel accordingly.

[carlpeterson]

All suggestions listed are good. I have a Safari Simba with a 8.1-liter gas engine, with a no-wind mileage of 9 mpg. I can not tell any diiference in the mileage when using the dash AC, whereas the generator uses about 1 gal. per hour. Don't drive faster than 60 mph, don't accelerate rapidly, and be aware of traffic conditions to minimize the use of brakes.

[wolfe10]

For an excellent factual discussion of the factors affecting performance and MPG, go to:

https://ohe.cat.com/cda/files/287140/7/LEGT5364.pdf

The document is "Understanding Coach (RV) Performance" and was written by Caterpillar Corp, but applies equally well to all engines, as physics is physics.

[mavric65@aol.com]

me my wife

I used to have a class c when my wife and i would head out for a long trip i would always drive 55 to 58 mph inturn i could usually get 10.5 to 11 mpg and she would always say your driving like a GRANDPA LOL!!!!!!!!!!! So on one trip from a place we often travel to i went roughly 70 to 75 mph and only got 6.5 mpg . It just takes much more fuel to push a box through the wind and i am in no hurry to get where i am going so in my new class a ijust put it on cruise at 58 and say woman be quiet LOL!!!!!!!!!!! Just kidding about that : The best way is to just slow down and enjoy the ride and scenery

Mike & Christina

[jimbrightly@worldnet.att.net]

Keep your eyes open for an upcoming article in FMC on MPG. The differences in MPG vs. speed is covered thoroughly, plus several other suggestions on how to improve or maintain your fuel mileage.

[DougC]

Hi Tom,

I just wanted to add my two cents that I tried to follow several suggestions on fuel mileage on the FMCA forum, and it seemed to help. We just concluded the trip

Total miles driven from Palm Springs CA to College Park MD, 2,863 miles, driven in 7 days or an average 400 miles per day. FYI, to have a more relaxed trip we concluded that we should have planned at least 9-10 driving days (300 miles per day average), with a rest day about every 4 days. That will be our planning technique when we head down the East Coast and then back to California later this month. But, we have had a good time getting here...

We got better than anticipated miles per gallon with the RV. I planned 6 miles per gallon and it appears we averaged 8-8.5 miles per gallon. We did follow a number of suggestions from one of the forums discussions regarding improving diesel fuel mileage. According to the Garmin we have, our overall average speed was 54-55 mph. In reality, I did attempt to cruise on the Interstate at 65 mph and did use the economy mode on the Allison transmission. Tires are at 120 psi front and 110 psi rear. And I worked at talking advantage of rolling hills (and there was a lot of them.) But I think keeping the highway speed at 65 mph had the most impact.

DougC

[tbutler]

Doug,

I've been following your trip on your blog. Thanks for the update on the fuel mileage. Now that you have picked up the toad, we'll find out how it affects your fuel mileage on the return trip! You may be able to offset the additional weight and its effect on your mileage with your anticipated slower travel. Enjoy the trip home.

[rsmolway@comcast.net]

We have a 33,000 lb GVW coach with a 300 Cummins ISC Engine. At first we kept it at 55MPH and achieved 10+ MPG. At this speed we turned 1400 RPM. At a Cummins seminar I was told that I could damage the engine if I travelled below 1650-1700. Know we speed along at 65+ and achieve only 8MPG. We seem to spend most of our time in the passing lane.

Did anyone ever hear of this advice or should I slow down a save a few bucks ??

Capt. Ron

[wolfe10]

We have a 33,000 lb GVW coach with a 300 Cummins ISC Engine. At first we kept it at 55MPH and achieved 10+ MPG. At this speed we turned 1400 RPM. At a Cummins seminar I was told that I could damage the engine if I traveled below 1650-1700. Know we speed along at 65+ and achieve only 8MPG. We seem to spend most of our time in the passing lane.

Did anyone ever hear of this advice or should I slow down a save a few bucks ??

Capt. Ron

Would love to see that statement in writing from Cummins!

Running below PEAK TORQUE RPM at more than light throttle can damage a diesel.

What is your peak torque RPM?

I would be surprised if your Allison ECU is programmed to allow you to be able to operate the engine below peak torque RPM anyway.

Brett Wolfe

[ITResC@gmx.com]

Would love to see that statement in writing from Cummins!

Running below PEAK TORQUE RPM at more than light throttle can damage a diesel.

What is your peak torque RPM?

I would be surprised if your Allison ECU is programmed to allow you to be able to operate the engine below peak torque RPM anyway.

Brett Wolfe

Brett, I have followed many of your very informative pieces, but I need some more information on the PEAK TORQUE RPM you refer to many times. Can you elaborate on it more for the uninitiated?

[wolfe10]

Brett, I have followed many of your very informative pieces, but I need some more information on the PEAK TORQUE RPM you refer to many times. Can you elaborate on it more for the uninitiated?

Cathe,

Each engine has an RPM at which it produces Peak Torque. It will be stated in the engine manufacturer's sales as well as technical sheets on the engine.

Some Aflas have Caterpillar C7's, some Cummins ISL's. I don't know which you have.

But here is the sheet on the Caterpillar C7: Caterpillar C7 Spec Sheet

This sheet lists Torque as 860-950 Pound- Feet (depending on how the engine is speced) AT 1,440 RPM. SO 1,440 RPM IS PEAK TORQUE RPM.

Peak torque is significant, as it also determines the LOWEST SAFE OPERATING RPM FOR THAT ENGINE at anything but light throttle.

And for most modern diesel engines, operating 150-200 RPM above that RPM will generally give the best MPG.

[Guest Wayne77590]

And a Quote from Cummins:

"The 6.7L Turbo Diesel achieves a peak torque of 650 lb-ft at 1500 rpm for the automatic transmission configuration and maintains that peak torque all the way to 2800 rpm......and a full 610 lb-ft peak torque for the manual transmission."

Maybe Brett can interpret that for me - I'm definitely not mechanically inclinded.

[wolfe10]

And a Quote from Cummins:

"The 6.7L Turbo Diesel achieves a peak torque of 650 Pound- Feet at 1500 rpm for the automatic transmission configuration and maintains that peak torque all the way to 2800 rpm......and a full 610 Pound- Feet peak torque for the manual transmission."

Maybe Brett can interpret that for me - I'm definitely not mechanically inclined.

Wayne,

Your peak torque RPM is 1,500. So you would not want to operate your engine below 1,500 RPM except at light throttle (and the Allison transmission should be programmed to do this). And 1,650-1700 RPM would likely give your best MPG.

The larger the engine, the lower the Peak Torque RPM.

Example Caterpillar C7 Peak Torque RPM is 1,440. C15 Peak Torque (2050 Pound- Feet by the way) is at 1,200 RPM.

Brett Wolfe

[jimbrightly@worldnet.att.net]

Brett, just a small reminder. Engine torque is given as pound-feet; wrench torque is given as foot-pounds.

Jim

[wolfe10]

Brett, just a small reminder. Engine torque is given as pound-feet; wrench torque is given as foot-pounds.

Jim

Thanks, Jim.

I went back and edited the torque units to be Pound- Feet, not Foot-Pounds.

Brett Wolfe

[Guest 2driftrs]

Brett,

We sure wish we had a diesel pusher, but we're stuck with our front engined gas class A unless we win the lottery. Would it be a good thing to start a separate topic for gas rigs? Some of the tricks and modifications we've made over the years fit class A and C gas rigs, but most don't apply to diesels, other than driving tips.

Barry and Jan

[wolfe10]

Brett,

We sure wish we had a diesel pusher, but we're stuck with our front engined gas class A unless we win the lottery. Would it be a good thing to start a separate topic for gas rigs? Some of the tricks and modifications we've made over the years fit class A and C gas rigs, but most don't apply to diesels, other than driving tips.

Barry and Jan

ABSOLUTELY. Just go to "Engines." Click on "New topic" and share your ideas.

My only reservation is that there are more differences in the gasoline drivetrains than between different diesel drivetrains.

The "old technology" big block push rod Chevy 454 and Ford 460 are very different from the "new technology" overhead cam high RPM Ford V10 6.8 liter.

And transmissions range from 3 speed with no lockup converter to 6 speed with lockup converter in 2nd, 3rd, 4th, 5th and 6th gear.

So, the more specific information you post AND the the more specific information you suggest others post, the more relevant will be the whole discussion.

Thanks.

Brett Wolfe

[Guest 2driftrs]

Brett, will do. This will be fun, as our first experience in RV'ing was in a borrowed Ford Econoline conversion van. We graduated to our own class C with a 350 Chevy, 4 bbl carb and 3 speed automatic. Then on to another class C with the 454 Chevy and a 4 speed automatic, and finally our Georgie Boy with the V-10 and a very sloppy Ford 4 speed.

Strange - - my 26,000# GVWR motorhome has a four speed while my 3,500# car has a six speed. I'm still toying with the idea of installing a gear splitter, but not sure I'll ever get back the investment cost. I'd love to hear from anyone with aftermarket overdrive/underdrive experience.

[pdhardin@gmail.com]

After reading these post, I got curious about the peak torque and fuel mileage on my ISC 8.3L 360 HP. My peak torque is 1050 lb-ft at 1400 rpm. I emailed cummins and this is the reply i received back from them.

You have contacted Cummins Inc. at our Customer Assistance Center in

Columbus, Indiana. This is our worldwide headquarters and has been our

home since Clessie Cummins founded the company, here, in February of 1919.

This year marks our 90th anniversary in business and we will continue to

work to provide the kind of products and service that will keep us in

business for another 90 years.

Peak Torque of 1050 ft-lb is reached at 1400 RPM, it would be recommend for

best overall performance and fuel mileage, to run the engine at approx.

1700 RPM.

On my next trip at the end of this month I will try running at 1700 rpm to see if I get better fuel mileage. I can maintain these rpm at lower speeds by locking out 6th gear if needed. I also realize that tire pressure, breaking and terrain come into play here. Thanks for the info that you guys provided. It got me curious. I'll let you know if I improved my fuel mileage.

Road Dog

[wolfe10]

Peak Torque of 1050 ft-lb is reached at 1400 RPM, it would be recommend for

best overall performance and fuel mileage, to run the engine at approx.

1700 RPM.

On my next trip at the end of this month I will try running at 1700 rpm to see if I get better fuel mileage. I can maintain these rpm at lower speeds by locking out 6th gear if needed. I also realize that tire pressure, breaking and terrain come into play here. Thanks for the info that you guys provided. It got me curious. I'll let you know if I improved my fuel mileage.

Road Dog

Road Dog,

Read VERY carefully the statement they give you. They did NOT, repeat NOT address what RPM to run the engine for best MPG. They gave an almost double talk answer of BEST OVERALL PERFORMANCE and fuel mileage.

You will get better MPG at closer to 1500 RPM if on relatively flat terrain (where it does not downshift on every upgrade). In rolling hills/mountains, you will need to run a little faster to get closer to "Best Overall Performance."

1500 and "economy mode" on the Allison will likely give you best MPG.

Brett Wolfe

[pdhardin@gmail.com]

Brett......Thanks for the info. I do use the economy mode when I'm not in the mountains. I did notice on my last trip when I was running 55 and locked out 6th gear that it had real good power. I don't recall what rpm I was actually running. I'm getting old. LOL

Thanks again for the info.

Road Dog

[Labrador47]

I agree with Tom. Your criuse speed has a lot to do with your fuel economy. The wind is beyond your control.

We are running a Country Coach powered with a Cummins ISX 600-1850 lb.ft. torque and weighing in at 51,000+ on the 'CAT' scales at the travel center with full fuel and water.

We tow a Saturn Vue most of the time. I found that running at 68-70 mph on the highway was delivering me 6.5-6.8 mph on average.

A head wind is a curse! It can rob you of another 10-15% of your economy at almost any speed. It does seem that life is a head wind mose of the time.

We decided that we would slow down a little and attempt to save some money by cutting our criuse speed to 63-64 mph on those long runs.

Well, we are now enjoying 7.3-7.6 mpg now on a regular basis.

That big flat front on our coach really plows thru the air like a brick. The higher our speed the more wind resistance created.

We take a little bit longer to get to our destination but it is worth it. With diesel over $5.00/gal last year we learned a good lession. $.70c a mile just for fuel wasn't any fun. besides, in the scary event of a front tire blow out, controlling the wagon is a bit easier at 60 that at 70.

Bill Herring

F122708

2008 Country Coach Affinity

ISX600/Allison MH4000

Naples, Fl.

Toad-Saturn Vue

[Guest Wayne77590]

We just completed about 600 miles round trip. After reading the information in FMCA Mag. I decided to check the theory (most likely fact). On past trips I ran about 64-65 MPH, with average much lower, pulling a 2007 Saturn Vue. My Fuel miles were 8.6-8.9.

So, on this trip I decided to do 55 MPH. I placed it on cruise control and it stayed there except when necessary to slow down, or going through lower speed zones. Nothing else changed except for speed and my MPG went to 10.4. Goin up I used the Interstate, and coming home I used Hwy-6

Since I took the toll way home from the North side of Houston, my wife started chiding me about my savings being spent on the toll road.

For a thousand miles, that is about a $40 savings at $2.50 a gallon. (That's a lot of ice tea)

[deenad]

Peak torque on the ISC is 1400 rpm, on the ISL's it's 1300.

We have the Banks kit on our ISC 350 and have 1200 ft lbs from 1400 to 1700 rpm, I still try to run around 1600 rpm even so. I also find that, with the much wider torque band I run in "economy" mode most of the time.