A/C and MPG

[Kuato]

Originally Posted By: Warstud
I decided the other day I was going to check my Mileage on the way to work with the A/C OFF. And compare it to the Mileage with the A/C ON... on my way home. BTW it's 35 miles to work. Here's what I got.

To work with the A/C off I got 22 MPG

To home with the A/C on I got 20.5 MPG

With the long commute...I was always trying to save money on gas but after this test maybe I'll go ahead and just use the A/C. Thoughts?


Any way you slice it, air conditioning requires power. More power required = more fuel used.

Air conditioning with windows up carries less drag than a/c off and windows down...BUT... That is speed dependent; above about 50mph is where the a/c on with windows closed starts to have advantage.

Edit: thought this had posted earlier but the pad must have waited for a new connection, so I missed things like

Originally Posted By: Shannow
http://www.motivemag.com/pub/feature/tech/Motive_Tech_Parasitic_Loss.shtml


Apologies for the late post

 

[bbhero]

I would suggest wind can be a FAR bigger factor than a\c being on or off. Obviously traffic patterns,topography, air temp and humidity are big factors as well. Mythbusters did a part of their show on this and they found the a\c on was more fuel efficient than windows down. Where a\c really hurts fuel economy is from dead start. Losing twenty to thirty of power there does make a big impact no doubt. Once at cruising speed a\c becomes advantageous obviously due to the lack of drag.

 

[raytseng]

Originally Posted By: Shannow
Link I posted shows the power loss that the ancillaries use, and it's not insubstantial.

If if it only takes 20-30hp to motor, the losses are quite large.

however, that's not what the OP was asking. He's asking about MPG, where you're not usually looking for max HP.

If you look much further to the left of the horsepower charts which is where you're typically driving, the losses are not that large.

 

[Lyman]

I get 23 mpg on the way to work and 19 mpg on the way home. Oh and I run the AC both ways. Yeah I live at 1800ft and work at sea level. Too many factors at work here to try and figure out how much your AC is hurting your mileage.

 

[gfh77665]

Temps 95-100...humidity 70-90%...heat index 110-115.

 

[Shannow]

Originally Posted By: raytseng
Originally Posted By: Shannow
Link I posted shows the power loss that the ancillaries use, and it's not insubstantial.

If if it only takes 20-30hp to motor, the losses are quite large.

however, that's not what the OP was asking. He's asking about MPG, where you're not usually looking for max HP.

If you look much further to the left of the horsepower charts which is where you're typically driving, the losses are not that large.


I'm not stating, nor looking for max hp, it's a power curve, and the ancilliaries take power to operate.

Take the VW...what's the gap at 2500RPM ?

5hp ?

Take the corvette, what's the gap at 2500 RPM ?

Line is quite steep, but if you use the torque curve instead, it's 9hp.

If the power required to drive a car down the road at 60MPH is around 30, and you give up an extra 3 hp to drive the AC, then you are going to use around 10% more fuel to drive the vehicle with the AC on.

If you are driving around full throttle, the AC will use a much lesser percentage of the fuel burn...

make sense ???

 

[cazi]

So as a rule of thumb; the bigger the car, the smaller the differrence.
To determine precisely would be pretty complex (load/BSFC/aero/gear ratio/etc. and loads of variables).

 

[fdcg27]

There must be some loss in fuel economy since it does take some power to run the compressor.
The loss appears pretty nugatory on a tank by tank basis.
I've had very good tank fuel economy numbers using the A/C a lot and sometimes inferior numbers while thinking that I'm being virtuous and avoiding its use.
Using it on a regular basis is probably key in keeping the system functional and leak-free.
Anyway, the A/C is there for your comfort, not as a fuel economy enhancer, so use it whenever you want.

 

[JustinH]

Not a lot of options when it is 105 degrees in Texas.

 

[Eddie]

Mazda CX5. I've kept a running measurement of the AC on 100% in summer vs OFF during one winter here in Florida. I did the measurement over 2,000 miles for each value. My conclusion yielded, that the AC on 100% resulted in o.94 MPG increase when used as a daily driver in a suburban setting. Maybe modern AC units have increased in efficiency over those of 10 years ago.

Ed

 

[IndyIan]

Originally Posted By: Shannow
Link I posted shows the power loss that the ancillaries use, and it's not insubstantial.

If if it only takes 20-30hp to motor, the losses are quite large.

Bombing along at 100km/h on a flat road, the Focus loses about 10% of the instantaneous mileage when the AC cycles on. I'm guessing the compressor is on about 1/2 the time, so for my commute having the AC on uses maybe 4-5% more fuel.
For compact tractors, the rule of thumb is that A/C in cab uses about 5 hp, so if the tractor is near its limits to run a piece of equipment and then you want a cab with AC, you should probably get one a 5hp or so more powerful.
For my commuting, AC isn't really needed as its cool in the mornings and if I sweat a little on the way home it doesn't matter, but for family trips on faster roads its a necessity then.

 

[HTSS_TR]

Originally Posted By: Shannow
If the power required to drive a car down the road at 60MPH is around 30, and you give up an extra 3 hp to drive the AC, then you are going to use around 10% more fuel to drive the vehicle with the AC on.

I observed about 3-5% worse MPG with A/C on while driving on highway at around 75-80 MPH.

When ambient temperature is around 60F or lower MPG of my E430 is around 24-25 MPG with A/C off, when it is above 70F gas mileage went down to 23-24 MPG with A/C on.

I think you're right, with lower speed at around 60-65 MPH the effect of A/C on fuel economy may be higher.

 

[bulwnkl]

My Honda Ridgeline loses 5-10% when it gets hot enough to run the A/C all the time, vs. when I don't need to. Remember that the cooler weather also results in lower oil temps, which of course reduces mpg. So, the mpg difference between A/C & no A/C would actually be a bit higher if I was operating in 105 - 115F (actual temp, not 'heat index' nonsense) in both cases.

While I've run down the road in 115F weather without A/C before, it's VERY unpleasant in a car, and I'm not doing it for a test like this.

 

[blupupher]

Did a little "research" on my 2004 Elantra on this very topic when I was driving 300+ miles a week.
Over about 6 months I checked my MPG to and from work. Same route each day (53 miles, 50 miles on highway), similar traffic, same speed (55 mph), only difference was A/C on or A/C off with windows down.
A/C on, 39 mpg
A/C off with windows down (and sunroof open) - 41 mpg.

2 mpg may not seem a lot to some, over the long haul, it makes a big difference.

There are more variables in the long run, I don't drive like this all the time, and my "test" was done in the early am, so it was not too hot. If it is hot, I choose to use A/C.
I have found in similar driving in my truck, it makes no difference in MPG between A/C on and A/C off. I think the smaller engine on the Elantra took a bigger hit on mileage than a large V-8 (just as others have said above).

 

[lenjack]

Too small a sample size. Try it roundtrip with it on and another run with it off. Better yet, do a 3 day test.

 

[Gasbuggy]

I was into the ecomodding thing for a while and consumption between vehicles will vary because A/C compressors consume different amounts of power. And the differences can be considerable. I believe someone on ecomodder.com compiled information about different compressor consumption.

My CTS-V has a very efficient Denso compressor and on the highway when I turn the A/C on it does not impact my instant MPG. I can barely feel it come on. I believe the loss is around .5 mpg. If this car had one of the GM variable displacement compressors it would feel like I dropped a small anchor on the ground. I've averaged a hair over 30 mpg on a full tank in this cammed CTS-V before and have done shorter trips at 33mpg.