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Old 06-25-2013, 10:24 PM   #1
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MPG Using AC

I know this has come up before but has anyone done a real analysis of using the dash AC -vs- running the generator and the coach AC on long trip? We are headed to Montana this summer and I would like to use the most economical method to stay comfortable during the 1800 + mile trip. I suspect the generator and coach air is the answer but I would like a solid comparison if available. Both RV and generator are diesel.


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Old 06-25-2013, 10:40 PM   #2
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Most gen sets will use 1/2 gal per hour at full load. Dash A/C compressor takes approx. 11HP when it is cycling. I'm not sure if anyone can give an exact cost figure one to the other .
The general feeling from the last time I saw this posted was, use the dash A/C and if it can't keep you comfortable fire up the gen set, some were saying that using the rear roof air ,not only helped spread the cool air throughout the coach and kept the load and fuel use of the gen set down.
JMHO. for $2 an hour I'll spend that to be comfortable.

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Old 06-26-2013, 04:57 AM   #3
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We run both. I don't find much of difference of running one or the other, both, or none. Obviously running the gen will burn a little fuel but we found that running both keeps both from running constantly and in my mind reduces each to some degree. We also tow most of the time and I don't see any difference in power running the dash AC or not running it. But we also travel with two big dogs so I don't want the coach getting hot. Although we do run with the back half of the coach closed off (bed/bath) most of the time and only run the front AC. If we didn't have the dogs I would hang a curtain behind the front seats and run with the dash AC only until we were within a couple hours of stopping for the day then turn the roof AC's on.
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Old 06-26-2013, 05:53 AM   #4
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I get about 8 mpg on avg going about 60 mph. The few times I checked the mpg with the AC on, my Scanguage said a drop of about .02 mpg. Using $4 @ gal, that's $30 per hour vs $30.76 per hour with AC on. Even if mileage dropped to 7.5 mpg, that would only be $32 per hour.

My genny burns 1/3 gph with no load and 1/2 gph with a decent load like one AC. So it's clear to me it's cheaper to run the dash AC, if choosing one over the other.
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Old 06-26-2013, 07:02 PM   #5
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I have an 8 KW Onan. The manual says it burns:

- .13 GPH at no load.

- .49 GPH at half load.

- 1.02 GPH at full load.
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Old 06-26-2013, 07:13 PM   #6
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I typically run the genny and the roof ac. My dash air doesn't have enough gusto to keep it cool enough running here in Georgia. Roof ac works great.
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Old 06-26-2013, 07:32 PM   #7
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i have run both
and if I get warm up front i have no problem with both running

that being said
.5 to .75 gal burned per hour for the genny to run both a/c in the freeze you mode is good for me
i set the rear one on 72 auto and the front 77 low fan
i also use the 12v bus fans up front to move some air if needed
i say let your internal temp and comfy level be your guide
but driving into the afternoon sun......nothing stops that big heater from zapping you, i have run my sun shade down pretty low to block alot of sun
it does help,
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Old 06-26-2013, 07:42 PM   #8
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Originally Posted by Mrchips View Post
................... I would hang a curtain behind the front seats and run with the dash AC only until we were within a couple hours of stopping for the day then turn the roof AC's on.
That is exactly what we do. It works very well adn I can't verify that I burn any more fuel that without the A/C. So, IMHO dash A/C fuel burn is negligible vs. generator.
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Old 06-26-2013, 07:44 PM   #9
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When the outside temperature gets hot and humid enough that I am not able to stay comfortable inside while driving, the generator will come on and I will run either one or both A/C's along with the dash air.

I could care less what it is costing me in MPG. I want to be comfortable and not irritable when driving.

I only get 6.5 - 8.5 depending on whether I am hauling my trailer or towing just the car, or just using the coach plus it also depends on what type of terrain I am driving over.

At those rates per MPG, who cares!

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Old 06-26-2013, 08:30 PM   #10
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On a 30,000 pound gross weight vehicle, the energy used to run the air conditioning is negligible, compared to the task of moving the rig, which ever way you do it. It will be somewhere between 6.5 and 8 mpg and I doubt you could see the difference.

Is that really an issue?

On our much smaller rig, with the wimpy F53 dash air, we would probably run the generator if the ambient temp was really high. Since we're normally running in the coastal areas of the western states in May/June, and late August, September and early October (we stay home when the rug-rats are camping) it's not a big deal for us. Whatever is comfortable and I haven't been able to see a difference in MPG which ever way.
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Old 06-26-2013, 08:51 PM   #11
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On a sunny summer day in Texas, you need to shed every BTU of heat you can, any way you can until you get to the beach & jump in the Gulf, or a lake with water in it, if you can find one.
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Old 06-26-2013, 09:24 PM   #12
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If it's hot, I want every last BTU of cooling power that I can get. I run the dash air on high and the generator runs the coach AC units. Our comfort is worth more than a couple bucks worth of diesel fuel.
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Old 06-26-2013, 10:36 PM   #13
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I'm with you Rich! You can't go anywhere in the Deep South in the summer and be comfy without the coach air running full time.
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Old 06-26-2013, 10:46 PM   #14
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I find using the dash air blows on my leg regardless of where the vent is pointed so I use the coach air which blows gently around me. As far as MPG, I would think that using the engine air would take hp, reducing performance and reducing MPG.

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