Gas Mileage re: Air Conditioner usage


#1

my wife read recently that you get better mpg with air conditioning running than with windows open and air conditioning not operating because of aerodynamic benefits. Is it true and, if so, at what mph speed is the break-even point?


#2

If you believe people on the internet, the speed is anywhere from 40mphto 60mph, depending on the type of vehicle. You would have less of a gas mileage reduction with open windows in a boxy old 70s Suburban, than say, a slippery bubble-shaped 1996 Taurus. You’d notice it more in the Taurus because the mileage is so high to begin with. You’d notice it less in the Suburban because all you’re focusing on is the black hole that is the gas filler neck.

On a more personal note, I found that in summer driving there was little appreciable difference between driving a car on the highway with the AC running and driving a car on the highway with the windows down and the AC off. Testing is not up to any kind of standards, but the car was a 1988 Buick Century sedan, 4-cylinder, automatic, lockup converter solenoid unplugged. I averaged 23.3mpg in mixed driving during the weeks the AC was working (and I used it ALL the time)and 23.1mpg during the weeks following an A/C compressor seizure. Not a big difference to me, but 0.2mpg would mean more to a Suburban owner for example.


#3

Mythbusters just did a segment on this and all I remember was the break point was about 50 mph.


#4

In general, at city speeds, I would say 45 mph and below, you can use the windows. At highway speeds, use the air conditioner because that will use less gas than windows at highway speed because of the wind resistance. Because of the wind resistance, you actually use more gas at highway speed if you use the windows than if you use the air conditioner.


#5

If you’ve got a dinky 4-cylinder, a car full of people / stuff, and a headwind, I think it’s fair to postulate that the AC can be less efficient. I have a Tbird whose AC system is not terribly healthy. It’s been converted to R134 from R12 and is prone to abnormally high system pressure at cruise. It puts a huge drag on the motor and I pay in MPG and power vs. driving with the windows open.


#6

I’ve tracked every single tankful of gas since I bought my Hyundai Elantra a year and a half ago, over 30,000 miles. Air conditioning knocks my mileage down about 2 mpg. Running with the windows open has no effect on mileage (i.e., I get the same mileage as running with the windows closed and no AC running).

Your mileage, of course, may vary, but that is one data point.

I recommend you do the same- drive both ways on your normal drive, maybe 5 tankfuls with windows down, and five tankeful with windows up and AC running- to determine what the effect is on your car.


#7

Go with the suggestion to try 5 tanks with and 5 without. The only definitive way is to test it for yourself.

That said, I find in my small 4 cylinder sedan, up to about 55/60mph AC reduces efficiency, and above it’s better to keep the windows closed.

But I think regardless of vehicle, if you’re driving in urban or suburban (less than highway) areas, it’s always better to drive without AC - from a fuel economy perspective.


#8

Mythbusters did a segment on this.Myth busted! Gas mileage was better at all speeds with the AC off and windows open.


#9

Just as a note, Mythbusters did this test twice and that is why were two apparently different answers. The test procedures were different and thus the different answers.


#10

Has anyone also considered the effects of wind on the A/C? It would help cool the condenser, thereby reducing high side pressure and reducing power usage. The condenser fan might slow down or stop if there’s an electronic pressure control, and the compressor can reduce its capacity if equipped.
On a side note, I have wondered if it’s more efficient to set the evaporator fan on high and use the thermostat to regulate temperature or set it low for better dehumidification. I think the answer will depend on whether or not the compressor is variable capacity. (A variable capacity compressor can slow down to prevent freezing and to regulate temperature, while the more common single speed compressor must turn off for defrost cycles at low speeds and to regulate temperature.) But although setting the fan on low reduces fan power usage, the evaporator will be colder, requiring defrost cycles and causing a theoretical drop in efficiency.
Maybe someone can connect a power meter to the entire HVAC system and let us know!


#11

An old Straight Dope column covered this question… interesting reading even if the answer isn’t definitive:

Russ


#12

In my Vic and Caddy too, I can drive with the windows up and the heater controls set on “vent”. This provides adequate ventilation without running the A/C or putting up with the wind noise… This configuration no doubt provides maximum mileage…


#13

Star882 has suggested that changing some of the variables in the A/C system’s setup might help efficiency. It seems to me that this is the kind of optimization that microprocessors are good at, and makes me wonder why this has not been done by the car makers already. Sorry, I’ve started off the topic now.


#14

This has been studied, tested, written and argued about for as long as I can remember. Most of the history suggests that in most cars at highway speeds (65-70) you get slightly better mileage with the windows rolled up and the AC on. At lower speeds you get better mileage.

In a convertible on a beautiful day you get better mileage with the roof up too, but why would anybody do that? Enjoy life. Do it the way you enjoy the most. This is not a rehersal.


#15

Ive got a Chevy Celebrity, 3.1 V6, very torquey engine. My mileage doesnt decrease at all with the AC on, I get 30-32 on the highway regardless. I can also climb almost any hill in overdrive.

I would think a less torquey engine would have more of an effect on mileage with the AC on, dragging it down more and making the car downshift more often.


#16

Prius is equipped with an electric compressor for its A/C and I believe it has similar functions.


#17

It really has a lot to do with the vehicle. Performance of AC, Engine performance, Aeordynamics of vehicle. I don’t think you can make a blanket statement either way.


#18

One of the things that complicates matters is that when temperatures are high, the air is less dense and wind resistance/drag on the vehicle is less, irrespective of whether the AC is on or not.

In my experience with a 96 Camry V6, I get ~27 mpg in hot weather (80+) with the AC on, and ~ 25 mpg in cold weather with the AC off.

Like they say, your mileage may vary!


#19

For some reason, advanced controls are not very well accepted in HVAC. Digital thermostats and electronic air handler controls are common, but only a few condensing units include variable frequency drive (VFD). As an electrical engineer, I don’t really understand why as the electronics needed for even a 5 ton unit are only a tiny fraction of the system cost and will very quickly pay for themselves in energy saved, as well as reduce noise and improve temperature and humidity control. Instead, units with variable speed compressors (like Carrier Infinity) are so marked up that they barely pay for themselves in their useful lifetime.
Of course, some cheap units don’t even include pressure controls or a proper expansion valve. They like to cut corners just to please customers…