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Dash A/C vs MPG … Fact or Fiction?


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Hi - I have been told that it’s more economical to run the generator and use roof A/Cs rather than the dash A/C when traveling. Does anyone have some insight?  
 

Thanks!

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Depends on which genset you have.  A Onan 7.5/8.0 are variable speed gensets.  The 10.0 and 12.5 are constant speed gensets.  Then there is the comfort issue.  IMO, half the fun is driving to your new location.  Then there is the DW complaining about how hot it is.  All that for a few gallons of diesel fuel???  Chuck B 2004 Windsor

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In my opinion there is zero loss of fuel mileage with dash A/C on a adequately powered Diesel engine. My coach only varies about 1.5-2 mpg when towing my race car trailer that is approaching 10k lbs. so the 5 horsepower it might take to drive that a/c is nothing

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Thanks for the replies! I have an older 10kW genset. My coach has a Cummins M11. 

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2 hours ago, CMillet86 said:

. . . .  so the 5 horsepower it might take to drive that a/c is nothing

Torque man, torque!  . . . but it's all ball bearings these days anyway (quoting Fletch).

My genset uses about 1/2 gal / hour.  The dash AC is insignificant.  I run the dash AC as much as possible because #1 I'm a cheapskate and #2 I'm afraid of wearing out the generator (yeah I know, LOL).  There's the noise from the gen & roof AC also. 

- bob

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1 hour ago, cbr046 said:

Torque man, torque!  . . . but it's all ball bearings these days anyway (quoting Fletch).

My genset uses about 1/2 gal / hour.  The dash AC is insignificant.  I run the dash AC as much as possible because #1 I'm a cheapskate and #2 I'm afraid of wearing out the generator (yeah I know, LOL).  There's the noise from the gen & roof AC also. 

- bob

Light on the gen start / stop switch is the only way I know my gen is running while travelling down the hiway!

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4 hours ago, Joel & Susanna said:

Besides, with the genset running, your not burning LP for the fridge and the wife can then fry or grill you a 60 mph hamburger lol.

I cherish my wife enough that I require her to be under a seat belt while the coach is in motion.  It's state law in many states.  Chuck B 2004 Windsor

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When im in route, I run the dash a/c all the time just cause either my hands or feet need cool air especially in rush hour traffic... (wish they offered seat cooling), plus since I serviced the dash a/c it flat gets cold on the long runs, I actually have to turn the temp knob warmer or change the dash setting to the defrost mode and just keep the cool breeze within the driving area without blowing directly on me.

As for the genny on enroute as long as I keep a nice air flow with a few open windows and the fantastic fans on its comfortable, however if its getting too hot I will run the genny and the a/c units, my owners manual I believe stated it will use 0.9 gallon per hr when under full load so I always keep that in the back of my mind.

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Yep, extra summertime propulsion engine fuel usage is from the fan. Compressor eats 5 max HP which is negligible. Now if you've got a side condenser a/c that runs the fan on high speed all the time, that would be tough to call. 

Edited by trailmug
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We run both when necessary otherwise it is just the dash AC or Heat.

We will also run the Aqua-Hot for extra heat when needed but that uses the engine heat to keep it warm.

We like riding  in comfort versus being concerned about how much we are spending in diesel.

Edited by Dr4Film
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I remember years ago in the mid to late 70’s when people discovered car mfgs designed the AC compressor to run in EVERY position except vent an floor… there comments then were the same now, that they felt it was a waste of fuel mileage, of course back then many engines were under powered (chevy Vega, ford pinto) so some of there complaints were valid. Then you also had the daily drivers that drove there cars like they were in NASCAR…

Now today they have become commonplace, that out here in Ca for the most part you have alot more road noise from bigger traffic patterns, along with homeless at stoplights, or the more than occasional traffic jam in high heats. To those folk i say “you go on ahead, turn that gas guzzling ac off” roll down that window an let the wind blow thru your hair! 
Good god nurse! I learned along time ago there are folks that will step over a dollor to pick up a dime!!! My rig weighs over 35k Lbs, the fuel i use to break the rolling resistance could run a Prius a long way… Honestly im not trashing peoples choises, yes the system takes energy to run it. Full stop! End of story! Can the amount of fuel required to run the ac, maybe, but who cares!!! I remember in the late 70’s when the DOT mandated all over the road trucks (18 wheelers) will no longer be available with out AC. Stating ac reduces stress, hang out sometime in a truck stop an see if you can find any of them that are NOT running there ac! I can go on an on however i really feel this point is Moot!

1 minute ago, Rikadoo said:

I remember years ago in the mid to late 70’s when people discovered car mfgs designed the AC compressor to run in EVERY position except vent an floor… there comments then were the same now, that they felt it was a waste of fuel mileage, of course back then many engines were under powered (chevy Vega, ford pinto) so some of there complaints were valid. Then you also had the daily drivers that drove there cars like they were in NASCAR…

Now today they have become commonplace, that out here in Ca for the most part you have alot more road noise from bigger traffic patterns, along with homeless at stoplights, or the more than occasional traffic jam in high heats. To those folk i say “you go on ahead, turn that gas guzzling ac off” roll down that window an let the wind blow thru your hair! 
Good god nurse! I learned along time ago there are folks that will step over a dollor to pick up a dime!!! My rig weighs over 35k Lbs, the fuel i use to break the rolling resistance could run a Prius a long way… Honestly im not trashing peoples choises, yes the system takes energy to run it. Full stop! End of story! Can the amount of fuel required to run the ac be measured maybe? but who cares!!! I remember in the late 70’s when the DOT mandated all over the road trucks (18 wheelers) will no longer be available with out AC. Stating ac reduces stress, hang out sometime in a truck stop an see if you can find any of them that are NOT running there ac! I can go on an on however i really feel this point is Moot!

 

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On 9/20/2021 at 10:41 PM, We2dynasty said:

We have a 350 hp cummins and when we run the dash air it does not affect fuel consumption at all.  When we run the roof air off the generator, there is a noticeable drop in mileage. Ed 2000 Monaco Dynasty

My SilverLeaf calculates miles per gallon used versus miles per tank. I always fill tank if close to the halfway mark especially if we are stopping for the night.

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On 9/20/2021 at 10:30 AM, Joe Lee said:

Hi - I have been told that it’s more economical to run the generator and use roof A/Cs rather than the dash A/C when traveling. Does anyone have some insight?  
 

Thanks!

That is a funny statement when you consider what it takes to turn six or eight 22.5" tires with nearly 50,000 pounds riding on them.

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  • 3 weeks later...

We have the gen running all the time while underway for the refer, so we run the rooftop A/C's as well.  Economical??  A 50k pound beast being fed diesel is a hungry beast to begin with, I don't think I'll worry about dash A/C, which we do use until the heat overwhelms it, or roof A/C, about which is cheaper to use.  Comfort first, it the reason we bought the beast.  I'll pay the fuel any day to avoid lugging the DW's 80 pound suitcase into a hotel room.  I have a bad back.  And I'll be damned if I want to take off my belt, shoes and empty my pockets to get on a plane.

Woody Miller

09 Dynasty

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Woody you should get the post of the day if we had one.  👍   

We could do pretty much the same thing in a bumper pull and do it a whole lot cheaper.  However this is what we wanted so we planned and can so here we are.            I've bought fuel a lot cheaper and a lot higher,  ain't figuring on stopping either,  might slow down some though.   Wife says I've already done that as much as possible.

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