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Old 08-26-2016, 01:30 PM   #1
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MPG & Air on/off?

Never owning a MH before I guess this question never occurred to me, but talking to a salesman it came up.

I assumed that while running down the road you would run the Fords (gas V10) air conditioner if that was really all you needed to stay comfortable. His advise was to generally always keep the engines air shut off, and instead run the gen set and coaches air units. The reasoning was that based on the amount of gas the gen set would use compared to the MPG decrease by running the engine air, you would save fuel.

What say ye experienced gas MH users? I know in my 1-ton diesel it wouldn't make much difference, but maybe this makes sense with a gas engine?
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Old 08-26-2016, 01:36 PM   #2
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that makes no sense, and he's probably not in the business of actually using a motor home very often...
actually, you can do either, or even BOTH, if it's really hot!

I don't think using the chassis air conditioner makes too much of a difference on the engine fuel usage, but the GENERATOR definately will... I think his advice is right the opposite of reality!
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Old 08-26-2016, 01:48 PM   #3
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I have seen several studies in motorhome mags that say generator option is not a good choice - unfortunately I can't come up with them. But, I doubt if you could find the mpg difference ,AC or not, in a motorhome. My Vegas is very comfortable in very hot weather running only cab air - have never considered turning on the roof air ( my previous 40 ft diesel pusher was a different story ...). I also know- in a modern car, running the AC is actually more economical than driving with the windows open ( aero drag increase). But not really on topic to this conversation....
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Old 08-26-2016, 01:55 PM   #4
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If you compare the fuel usage of the generator vs the V-10 you get something like:
Generator will use about 1/2 gallon of gas per hour (1/2 load, from here: https://power.cummins.com/sites/defa.../F-1123-EN.pdf ).

RV going down the road will get about 8 mpg @ 70mph (roughly) which is 8.75 gallons/hour.

If we assume a .5 mpg hit by the A/C then 7.5 mpg @ 70 mph is 9.33 gallons per hour.

Note that there are a lot of assumptions above (especially that I don't think the A/C gives you a .5 mpg hit..I bet its more like .2 or less--something not noticable). Another one is: I don't think the A/C will load the generator up to 1/2 load either.

Doing the math for .2 mpg hit: 8.9 gph.

I'm going to conclude it is a wash: Do either as you like; its not going to change your very large gas bill by enough to be noticeable.

I run with only the front unless its really hot, or a passenger on the couch complains that they are hot.
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Old 08-26-2016, 02:13 PM   #5
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I don't worry about MPG, when its hot I run the Generator and both AC's with the dash air.

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Old 08-26-2016, 02:27 PM   #6
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I do agree with the last post. If you are really concerned about .5mpg loss on a MH that gets 6-8mpg, you may want to second guess your purchase on something more fuel efficient.

Its always HOT here. I will run my genny for coach ACs and my dash AC if it is hot. My calculations always put me right at 7-7.5mpg running 65mph on my coach. Im happy with that.
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Old 08-26-2016, 02:39 PM   #7
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With all the motors doing like running a giant alternator pushing a huge sail (motorhome) its not gonna make a bit of difference.

Now heat on high on the other hand can make a difference in the winter if its really cold out. Need to block some of the air going to the radiator so the motor can run at optimum temp otherwise they run rich trying to warm up the motor.
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Old 08-26-2016, 02:59 PM   #8
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When people ask me how many MPG on get on the coach I tell them you don't buy an RV if your worried about MPG.
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Old 08-26-2016, 03:18 PM   #9
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I recall an episode on the tv show myth busters, they wanted to know if gas mpg was affected by the AC on a car. They used a car on a controlled track and ran several laps with the AC off and windows closed. Then ran laps with the AC on and windows closed.

Lastly they ran laps with the AC off and windows open.

There was no real noticeable difference with the AC on or off, they were able to conclude that running with the windows down increased some drag and affected the mpg slightly.

I think the conclusion was newer more effecint engines don't burn much more fuel with the AC running!
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Old 08-26-2016, 03:24 PM   #10
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Quote:
Originally Posted by Kev View Post
When people ask me how many MPG on get on the coach I tell them you don't buy an RV if your worried about MPG.
Kev
Kev, I agree it doesn't matter to many owners, but not all feel the same. I certainly consider fuel economy for various reasons. Mostly I prefer efficiency in most things so I personally like an RV that is just large enough and no larger. Others can drive 6 to 8 MPG motorhomes if that's their thing, but it's not mine. I know many owners who feel the same way.

A few years ago when gas was $4 a gallon or higher, some used lots were overflowing with large RVs, so it does make a difference to many owners.
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Old 08-26-2016, 03:38 PM   #11
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Originally Posted by Joe-FL View Post

....cut...

What say ye experienced gas MH users? I know in my 1-ton diesel it wouldn't make much difference, but maybe this makes sense with a gas engine?
Strictly from a fuel economy standpoint, run the engine-driven AC instead of 2 roof airs powered by a gasoline generator. It's more efficient.

Not only would you use more gasoline, you will put more wear on generator that in the long run will likely cause you more incremental maintenance.


Unfortunately, you are not comparing likely scenarios. Many who run the generator with roof AC will also run the dash AC -- so you'll use gas even faster.
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Old 08-26-2016, 04:06 PM   #12
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I drive with coach air on, I want comfort, does not effect MPG and if it did I would still run it. When we stop it takes about 15 minutes to cool house no big deal because I am outside setting up. My MPG averages about 10 no matter where I am or what I'm doing.
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Old 08-26-2016, 07:07 PM   #13
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Originally Posted by Chance View Post
Kev, I agree it doesn't matter to many owners, but not all feel the same. I certainly consider fuel economy for various reasons. Mostly I prefer efficiency in most things so I personally like an RV that is just large enough and no larger. Others can drive 6 to 8 MPG motorhomes if that's their thing, but it's not mine. I know many owners who feel the same way.

A few years ago when gas was $4 a gallon or higher, some used lots were overflowing with large RVs, so it does make a difference to many owners.
Your not driving a rv with the Ford V10 then!! Ford V10 & fuel economy will never be used in the same sentence regardless of how you drive it, 6-8 mpg only, with or without the a.c.
With that engine you better never pass a gas station.
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Old 08-26-2016, 08:43 PM   #14
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Originally Posted by Travelin' Texans View Post
Your not driving a rv with the Ford V10 then!! Ford V10 & fuel economy will never be used in the same sentence regardless of how you drive it, 6-8 mpg only, with or without the a.c.
With that engine you better never pass a gas station.
I wasn't going to reply to this thread as everyone has, pretty much, covered all scenarios..
However.....
about 45 years ago we were at a conference and the keynote speaker mentioned he owned a motorhome ..
someone in the audience asked him what mpg he got...
his answer, remember 45 yrs. ago ... was..
if you must ask you cannot afford a motorhome !
That statement holds true today !!

AND...as an off the wall side note...in June when we were crossing southern Colorado, coasting DOWN Wolf Creek Pass, our OBD said we were averaging
14 mpg !! You know how long that lasted !!!
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Old 08-26-2016, 09:18 PM   #15
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Originally Posted by Travelin' Texans View Post
Your not driving a rv with the Ford V10 then!! Ford V10 & fuel economy will never be used in the same sentence regardless of how you drive it, 6-8 mpg only, with or without the a.c.
With that engine you better never pass a gas station.
I have averaged 10MPG for almost all the 15000 miles in our V10. That's been all over the west and the south. I drive 65 MPH but do hit seventy on calm days.
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Old 08-26-2016, 10:03 PM   #16
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Originally Posted by Travelin' Texans View Post
Your not driving a rv with the Ford V10 then!! Ford V10 & fuel economy will never be used in the same sentence regardless of how you drive it, 6-8 mpg only, with or without the a.c.
With that engine you better never pass a gas station.
The Ford V10 can actually be an efficient engine. A lot of claims on the internet come from people who don't know the difference between engine efficiency and poor fuel economy. I wouldn't believe them.

And yes, I have a V10 and can EASILY get 15 MPG. But its a small and light van that doesn't take much power going down the road. Now if someone has a 13-foot tall, 8-1/2-ft wide motorhome that weighs 25000 pounds, please don't blame the engine if 6 MPG is all they get.




Back to air conditioners.
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Old 08-26-2016, 10:16 PM   #17
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Well, it sure sounds like most would agree that it doesn't save you any fuel leaving the engines air conditioning off and running the coaches gen set with air instead.


I am used to high fuel fill ups and bad MPG. We owned a 40' motor yacht with twin diesel motors and a 300 gallon tank. It stung a little to pull into a marina and have $700 to $800 pumped into it. Pumping less that $200.00 worth of gasoline into an RV doesn't sound too bad.
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Old 08-27-2016, 12:37 AM   #18
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Running flat Florida with maximum AC used less gas than pulling a toad through mountains with no AC. To me, terrain matters more than AC or not AC. That Ford V-10 is a good puller but going upgrade at 40 MPH with the pedal on the floor throws economy out the window. No use sweating while it happens.
CasaLoca in Maine this month
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Old 08-27-2016, 02:54 AM   #19
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It all follows simple math. Efficient gasoline engines will burn in the range of 0.4 pounds per hour per horsepower, give or take a little.

If your engine makes 300 HP to crest that hill because you put your foot into it, it will burn approximately 120 pounds per hour -- on instantaneous basis.

And 120 pounds per hour of gasoline is 20 gallons per hour.

If your motorhome is only doing 40 MPH, then that 20 gallons per hour yields 2 MPG.


Basically, any modern gasoline engine that has to make 300 HP to maintain 40 MPH going up a hill won't do much better than 2 MPG. Some could do much worse under those power conditions, but upside is limited.

With present designs not many gasoline truck-ready engines can do much better than 0.4 pounds per hour per horsepower. And the Ford V10 isn't far from that.

The upcoming Ford 6.2L V8 should do a little better, but even if it is 10% more efficient (and I doubt it will be at full throttle), what does it really mean? That instead of getting 2 MPG up a hill we will get 2.2 MPG?
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Old 08-27-2016, 04:01 AM   #20
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Mpg

Quote:
Originally Posted by Travelin' Texans View Post
Your not driving a rv with the Ford V10 then!! Ford V10 & fuel economy will never be used in the same sentence regardless of how you drive it, 6-8 mpg only, with or without the a.c.
With that engine you better never pass a gas station.
Don't know where you get your stats but I get 10 MPG with my Vegas.

Kev
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