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Old 06-24-2016, 08:52 AM   #43
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Old 06-24-2016, 05:43 PM   #44
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Originally Posted by Mich F View Post
I've read that the SAE says that running your dash air decreases gas mileage by 5-10%. The generator which uses around a 1/2 to 3/4 a gallon per hr., is much more efficient, at somewhere around the equivalent of 100 mpg.

Can you please provide some backup for that?

A typical (Onan) 4K generator has a 9hp air-cooled and carbureted engine, a typical Class C has a water-cooled and computer-controlled EFI engine.

I find extremely hard to believe the chassis engine is less efficient than the generator's engine is, especially considering that when the rooftop a/c is not cycling the Onan's engine is just running for no reason. The chassis a/c on the other hand has a clutch which disconnects it when the a/c is not cycling.

As for purely physics, it will take a loooong time for someone to try to explain to me how the fuel load, I.e. fuel mileage, could increase on an engine pushing an 8' X 8' brick through the air at 60 mph could increase by 5%, let alone 10%, just by adding an a/c with a parasitic loss of 1.1 hp (and only under load) to it. Fuel consumption is pretty much linear to horsepower consumed.

For a 1.1 hp addition in load to add 5% to fuel consumption (load) would mean the Motorhome was only using 22 hp in the first place. Yeah, suuuure. Lol
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Old 06-24-2016, 05:54 PM   #45
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Can you please provide some backup for that?
How many links do you want ? Here's one.
Does Driving With The AC On Actually Use More Fuel?
The U.S. Dept of Energy says air conditioner use can decrease fuel economy by more than 25%
https://www.fueleconomy.gov/feg/hotweather.shtml
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Old 06-24-2016, 06:33 PM   #46
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FWIW I think that a careful reading of those reports will show small fuel efficient cars take a significant hit with the A/C. Large heavy flat bricks are already so inefficient that they A/C is in the noise.

Note, I am not saying that under laboratory conditions I could not measure it. I am saying that most folks will not notice it with normal driving.
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Old 06-24-2016, 06:52 PM   #47
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FWIW I think that a careful reading of those reports will show small fuel efficient cars take a significant hit with the A/C. Large heavy flat bricks are already so inefficient that they A/C is in the noise.

Note, I am not saying that under laboratory conditions I could not measure it. I am saying that most folks will not notice it with normal driving.
I'm with you on that.

I tried to figure out has milage with and without AC, using my Scangauge II, set to instant MPG.

The slight hills on the road made more of a difference then AC on or off.

Even on a long trip, I tracked it as best I could, and only saw maybe 2 tenths of a MPG.
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Old 06-24-2016, 06:59 PM   #48
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FWIW I think that a careful reading of those reports will show small fuel efficient cars take a significant hit with the A/C. Large heavy flat bricks are already so inefficient that they A/C is in the noise.

Note, I am not saying that under laboratory conditions I could not measure it. I am saying that most folks will not notice it with normal driving.
EVs/hybrids take a much bigger hit in fuel efficiency,as you pointed out. When you're talking a vehicle that gets say 8 or 9 mpg, a 5 or 10 % loss in fuel economy isn't going to be very noticeable.
My first MH, back in 1995, was a 22' Class C with a 5.7L engine. That thing would really struggle going down the road with the dash air on, particularly when towing a boat. The auxiliary cooling fan was constantly coming on. Turning on the generator and running the roof air instead of the dash air, helped tremendously in the way it ran.
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Old 06-24-2016, 07:07 PM   #49
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How many links do you want ? Here's one.
Does Driving With The AC On Actually Use More Fuel?
The U.S. Dept of Energy says air conditioner use can decrease fuel economy by more than 25%
https://www.fueleconomy.gov/feg/hotweather.shtml

LMAO. Yes, I read those links.

But I must admit, I stopped reading the first one though when I got to the "When you turn the AC on in your car, it uses energy supplied to it by the alternator." part. My engine powers the a/c directly, it's not an electrically-powered unit.

I dis however have a bit of a giggle at the second one when the data in the reference said "On-road and laboratory experiments with a 2009 Ford Explorer and a 2009 Toyota Corolla were conducted to assess the fuel consumption penalty associated with air conditioner (A/C) use at idle and highway cruise conditions." and "both vehicles consumed more fuel with the A/C on at maximum cooling load (compressor at 100% duty cycle) ...".


My Motorhome is NOT based on either a Toyota Corolla nor a Ford Explorer, nor would the a/c ever run at 100% duty cycle.


Those examples are kinda like saying a gas engine is more efficient than a diesel because a Mini gets better mileage than a transport truck.


As Abraham Lincoln once said, "You shouldn't believe everything you read on the Internet.".
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Old 06-24-2016, 08:52 PM   #50
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I really don't care if you feel you know more than the SAE or the Dept. of Energy.
In my experience my first MH was a dog with the dash AC running, while my current MH doesn't have any noticeable performance issues with the dash air on, but it does run hotter.
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Old 06-24-2016, 09:11 PM   #51
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We haven't spent any time in extreme heat yet - and have been able to stay more or less comfortable using just the dash air. If/when we find that we're uncomfortable - I'll have no qualms about firing up the generator.
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Old 06-24-2016, 11:51 PM   #52
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Not really Murf2..this is my 3rd Motorhome with no appreciable differences. All Class Cs.
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Old 06-25-2016, 06:02 AM   #53
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I really don't care if you feel you know more than the SAE or the Dept. of Energy.

I don't know more, but I find it amusing trying to draw a parallel between fuel mileage impact of a Toyota Corolla and a Class C.

Kind of like saying a mouse and an elephant are the same thing; grey, four legs, big ears and a tail.
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Old 06-25-2016, 07:26 AM   #54
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Originally Posted by Mich F View Post
How many links do you want ? Here's one.
Does Driving With The AC On Actually Use More Fuel?
The U.S. Dept of Energy says air conditioner use can decrease fuel economy by more than 25%
https://www.fueleconomy.gov/feg/hotweather.shtml
Mich F
Both of those articles provide excellent information about the relationship of A/C use in automobiles and automobile fuel millage.
However RVs are NOT automobiles.
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Old 06-25-2016, 10:11 AM   #55
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I have owned both a class C and class A Motorhomes. Our normal procedure is to run both dash a/c and roof a/c when needed. I use the generator almost the whole time we are driving to run the refrigerator, a/c's ( we have 2 roof units), the in motion satellite receiver, TV and from time to time the microwave. The dash a/c is used when needed to help cool driver area. The amount of fuel the generator uses is minimal and is not a big concern. The roof units will keep the coach and drivers area cool most of the time with no problems unless we are driving directly into the sun. So to answer the original question... I keep the Motorhome cool by using whichever a/c is needed. Running the generator to operate the roof a/c and running the dash a/c to help. Neither is right or wrong... Only a preference.
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Old 06-30-2016, 04:17 AM   #56
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Live in Texas,2015, Sunseeker 2700ds

This past weekend it was 95 degrees, 5 pax, dash ac kept up fine for the 3 hour trip. Same trip this past January, 38 degrees, 2 pax, no ac. less than 1mpg difference in fuel usage.

My father in law , had various types ( A & Cs ) MH from 1985-2010 and from what I remember you had to use the ceiling ac in all of them to keep comfortable while driving this time of year. So maybe today's units are better insulated ? more efficient chassis ac ?

bottom line if it were about gas mileage, we would all be driving electric cars and staying in pup tents
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