Myth or truth, running A/C in your car reduces gas mileage

[Jeffrey Hicks]

On most new cars it is not true. It was true on older cars when they first had AC but not anymore...

[Dean]

It will. It has to.


There is a belt attached to the compressor. When the AC is on, that belt is activated. It will pull some power from the engine that would otherwise be used to propel the car.

[Dean]

The semi-scientific TV show "Myth Busters" did an episode on this and decided that it's better to run the A/C.

YMMV

The only reason I posted was to use that last bit... again,

That's not complete. It's better to run the AC with the windows up than roll the windows down and AC off. This is at highway speeds. The increased drag from lowered windows creates more work for the engine to overcome than the belt to the compressor does.

To have the absolute best MPG, you would keep the AC off AND the windows up.

If you are just cruising around the neighborhood at 30 MPH, roll the windows down.

[SRanney]

It depends. I ran my own experiment several years ago and determined that for my 1995 Jeep Cherokee 5-spd manual, I got better mileage with the A/C on. Here's what you do:

1) Keep the windows down, the A/C off, and drive like you normally do;
2) Record mileage and gallons purchased every time you fill up for a few weeks or months;
3) Calculate MPG for each tank (ideally, you'd have 30 tanks worth, but realistically, five would probably be okay);
4) Repeat steps one through three, but this time keep the windows up, the A/C on, and drive like you normally do;
5) Compare the data sets from your control (i.e., no A/C) to your treatment (A/C);
6) Hopefully, your car won't have any nagging mechanical issues that could influence the outcome of this experiment.

When I quit smoking four years ago, I had the perfect opportunity to run this experiment. As a smoker, I never had the A/C on as it would require the windows to be up. When I quit, I gave myself the "present" during an Atlanta summer of having the windows up and the A/C on for the entire summer. As I had been monitoring MPGs since I had owned the car, I had a great data set to compare my treatment too.

Ultimately, I found that I got better gas mileage with the A/C on. While statistically significantly different, running the A/C made little difference in the thickness of my wallet.

I enjoy watching "Mythbusters", but I question the statistical validity of many of their "experiments". Still makes for a fun hour, though I can't stand the chick or the short robotics guy.

Steve

[Todd S. Malicoate]

Dean is correct...engaging the AC compressor (running the AC) causes the magnetic clutch to "suck down" on the compressor and spin it (instead of free-wheeling when not engaged). This takes power from the engine that would otherwise be used for moving the vehicle, and thus reduces gas mileage.

However, it is such a small difference it really doesn't matter. Be comfortable and run the AC. Even on your older vehicles. Trust your parts salesman...I've fallen asleep in the best, most informative AC clinics anywhere.

[windshieldbug]

... running the AC is not the issue.

Truth or Myth: does lowering the drag increase gas mileage?

Yes. So buy a low frontal area car, and close the windows.

Next question: Does running the AC create enough drain to overcome that advantage?

How fast are you going?
Do you have the recirculation on so that air is coming in anyway?
Do you have MAX AC on so that the car is using the compressor the whole time?
What is the outside temperature?
Do you smoke and leave the driver's window cracked?

Etc, etc.

I actually had a friend win a time-trial at Summit Point raceway in West Virginia because it was so stinking hot while we were lined up to go out, he turned on the air and rolled up the windows in his 911. He forgot to shut it off, but he was aerodynamically slick and mentally sharp while the rest of us were literally baking in our helmets and suits.

So like many things, it depends... (and yes, YMWV)

[Todd S. Malicoate]

Do you have MAX AC on so that the car is using the compressor the whole time?

The compressor still cycles, even when you select MAX AC. It actually only operates about 50-75% of the time, depending on need. MAX AC only fully opens the blend doors so that the maximum amount of cooled air enters the cabin.

I'm still going with my answer...it's such a small difference it doesn't matter.

To quote Mr. Spock..."a difference which makes no difference is no difference."

Todd S. "tired of explaining the same scenario to customers over and over" Malicoate

[Rick Denney]

At highway speeds, don't underestimate the effect of open windows on drag. The power required to propel a vehicle is something like:

P = V (Fw + Fr + Fg)

where P is power, V is speed, and the F's are forces. Fw is the force caused by wind drag, Fr is the force caused by rolling resistance, and Fg is the force caused by a grade.

Fw = 1/2 A Cw Rho V^2

where A is effective frontal area, Cw is the drag coefficient, Rho is air density (about .06 lbs/ft^3), the force of it which is found by dividing by g (which is acceleration due to gravity), and V is speed.

Fr = W Cr

where W is the weight of the vehicle and Cr is the coefficient of rolling resistance. (If you know the weight in kilograms, remember this is force not mass, so you have to convert to newtons by multiplying by g, which is 9.8 meters/sec/sec.)

Fg = W S

where S is the slope in percent.

Slopes and Cr are usually very small numbers, in the range of up to about 0.8% for rolling resistance and perhaps 10% for slopes. On a flat road, take the slope term and Fg out of the equation.

So, let's calculate the power required to move a 3000-pound car at 88 feet/sec (60 mph) down a flat road with no wind. Let's say the drag coefficient is .35 (good) and the car has a frontal area of 18 square feet.

The rolling resistance, Fr, will be 3000 X .008 = 24 pounds force.

The wind resistance, Fw, will be 1/2 X 18 X .35 X .0745/32.2 X 88^2 = 56 pounds force.

The power required to maintain speed will be (24 + 56) X 88 = 7,040 ft-lbf/s, or 13 hp.

A modern air compressor requires perhaps 5 horsepower to provide the typical 8000-10,000 BTUH of cooling capacity, assuming the AC is running at max. Since even on hot days, AC units in cars rarely have a duty cycle more than about 50%, that's 2.5 hp on average.

So, what change in the drag coefficient would be needed to add 2.5 hp to the power requirement? 15.5 hp is 8525 ft-lbf/s. Divide by 88 and then subtract 24 yields 73 pounds force for wind resistance. That would take a drag coefficient of .45 with other parameters the same.

Thus, the question for such a vehicle is: Do open windows change the drag coefficient from .35 to .45? Maybe. Maybe not.

It's not cut-and-dried. I suspect that it may go one way with one vehicle and the other way with another. A small, efficient vehicle would see a bigger change in drag coefficient from rolling the windows down, but the air compressor would be a higher percentage of the horsepower required to move the vehicle at speed. And, since the power requirement increases with the square of the speed, it will be much more likely that the AC will be more efficient than open windows at higher speeds. As the mythbusters people found, there was a speed above which it was better to run the AC, but that speed will be different for each vehicle.

It's too much to worry about, in my view. I'm more scared of hypermilers driving on grossly over-inflated tires than in spending an extra few cents per tankful because I run the AC. And I don't like sweating.

Rick "who doesn't want to spend in dry cleaning what he might save in fuel" Denney

[tofu]

--

[ken k]

On my four cylinder escort it definitely does. About a 10% change. i can get upwards of 32 mpg w/o the AC. 28ish with it. I recently pumped up my tires about 8 psi more (38 psi) and have not been using the AC much unless it is unbearably hot and then not driving over 55 to 60 mph and have been getting 35mpg. that adds about 50 miles to a tank of gas.

I think on larger engines like 6 or 8 cylinders it doesn't effect it as much since the engines are stronger. I never remember noticing the AC cycling on and off on my old Tauruses (Taurii? Taurae? whatever...) But I can really feel when it clicks on and off on my escort.

ken k

[tbn.al]

I recently pumped up my tires about 8 psi more (38 psi) and have not been using the AC much unless it is unbearably hot and then not driving over 55 to 60 mph and have been getting 35mpg.ken k

It's too much to worry about, in my view. I'm more scared of hypermilers driving on grossly over-inflated tires than in spending an extra few cents per tankful because I run the AC. And I don't like sweating.

Rick "who doesn't want to spend in dry cleaning what he might save in fuel" Denney

Subtract the savings on 100 tankfuls from the cost of repair after a collision caused by hyper inflated tires on a rain slick road and you are major screwed.

[windshieldbug]

Subtract the savings on 100 tankfuls from the cost of repair after a collision caused by hyper inflated tires on a rain slick road and you are major screwed.

OK; I'll bite:

Racing in the rain meant you put on well-treaded tires pumped way up to drain water from the treads.

On a "rain-slicked" road, I'd think it would be more tread-wear than over-inflation causing major copulation.

[ken k]

Subtract the savings on 100 tankfuls from the cost of repair after a collision caused by hyper inflated tires on a rain slick road and you are major screwed.

OK; I'll bite:

Racing in the rain meant you put on well-treaded tires pumped way up to drain water from the treads.

On a "rain-slicked" road, I'd think it would be more tread-wear than over-inflation causing major copulation.

I still use the AC when it is really hot, especially in the mid afternoon. But often in the evening and early morning it really is not that bad with the window down. I just think people automatically use it without a thought, same at home.

I only pumped the tires up to 38psi. Hypermilers tend to go up to about 45 or the max for the tire. I also pump them up when I autocross to get a stiffer side wall and better cornering traction.

no chance of copulation there (unfortunately)

ken "like elephant, I just wanted to type that word" k

[iiipopes]

I live in a state where the gasoline is state law mandated gasohol. I get $#!+ for gas mileage, regardless.

[TubaRay]

I live in a state where the gasoline is state law mandated gasohol. I get $#!+ for gas mileage, regardless.

Ah. But the more gasohol you burn, the more you save the environment.

[MaryAnn]

I wanna type it too!!!! But I'm going to put it in a minor key...

minor COPULATION!



MA

[Rick Denney]

Subtract the savings on 100 tankfuls from the cost of repair after a collision caused by hyper inflated tires on a rain slick road and you are major screwed.

OK; I'll bite:

Racing in the rain meant you put on well-treaded tires pumped way up to drain water from the treads.

On a "rain-slicked" road, I'd think it would be more tread-wear than over-inflation causing major copulation.

But those rain tires were made of moist bubble gum. The compounds were so soft that if the road dried you'd wipe the tread right down to the carcass in a few laps, assuming you could keep it from melting and sending you into the marbles. That effect overcomes what I'm talking about, and the hypermilers prefer hard compounds to minimize rolling resistance in any case.

The size of the contact patch is directly controlled by the inflation pressure. If you have a 1000-pound load on a tire, the contact patch will be 50 square inches at 20 psi and 20 square inches at 50 psi. It is true that the resulting friction force is not controlled by the contact patch, but rather by the normal force and the coefficient of friction. But that assumes that adhesion is linear, which it isn't. The small contact patch has less area over which to dissipate surface heat, and therefore will go non-linear (i.e., melt) when the rubber breaks down from heat quicker. Underinflation also causes heat, but that's heat back into the tread away from the surface as a result of hysteresis inefficiency caused excessive deflection through the contact patch (and high slip angles). Translated: Underinflation causes tires to get hot and are drag to increase. Overinflation causes tires to skid more easily especially with hard compounds and wet streets. Inflation that leads to even wear usually provides optimum traction in the widest range of conditions.

I also used high pressures when I raced cars on street tires, but the reason was to control the shape of the contact patch. The higher pressure would give me a patch that was wider and shorter (rather than longer and narrower), which would maintain proper tire geometry in hard turns. That would minimize wear on the corners of the tread, and performance and race tires are wide with square corners. Note that in round-tire applications, such as bicycle and non-fatty motorcycle tires, that does not apply--there are no corners on the treads.

There's a reason off-road drivers and skilled snow-drivers "air down" when driving in loose stuff. They need that contact patch as large as possible to provide traction rather than tearing the surface loose (for them it's the surface of the road or what's on the road, but for us it's the surface of the tire--but it's the same principle either way).

Another annoying trait of the hypermilers is that they coast down for about a mile approaching a stop to avoid using their brakes. I love it when someone driving a Pius--er--Prius does that--taking away much of the effect of regenerative braking. But it forces everyone behind them to slow down along the same coast-down curve as the hypermilers, which is always longer and more gradual for them because their tires are overinflated. That means everyone behind them is constantly cycling their brakes and throttles to maintain following distance.

But mostly its something that drives up the annoyance level of the typical psychotics behind the wheels of the surrounding cars, and that is exacerbated (no, that's not related to copulation) by the self-righteousness that the hypermilers exude. I wonder if they have figured in the cost of getting bumped into the ditch by a maniacal redneck in a Ford F-350 pickup.

Rick "finding balance between extremists" Denney

[TubaRay]

Doc (enjoying his AC today and every day)

Same here.

[SplatterTone]

Firstone Destination A/T tires. Got 'em from tirerack.com. I like them a lot. I've had them on the Toyota Tacoma for about 20K miles. Real good traction. They are holding up good for wear too. And on the A/C: I read somewhere that it makes about a 3% difference. Not enough to justify sweating and getting stuck to the seat.

[Rick Denney]

Firstone Destination A/T tires. Got 'em from tirerack.com. I like them a lot.

No doubt.

But there's just something about the BFG All-Terrain TA tires.

The Pass Patrol of Colorado recommended them exclusively for the jeep trails of the Colorado Plateau (which includes the most amazing jeep trails in the U.S., from above the timberline to desert canyon country). They were the ones that really knew how to rate jeep trail difficulty: Easy, Moderate, Difficult, and VDL.

I've had them on every off-road-capable vehicle I've owned, starting with my 1990 Cherokee. Survived the VDL-rated Pritchett Canyon Trail, the White Rim Trail, the Elephant Hill Trail, and about a dozen others in the Utah canyon country. Then, there was the '94 Grand Cherokee that had Michelins on it when I bought it. One of those tires couldn't take the relatively easy trail over the Henry Mountains, and I had to install a spare in unpleasant conditions. I replaced them. I have them on my current '90 Toyota Pickup, which has endured 190,000 miles of hell (but it's only been rolled once), and there's a fresh set on my '95 T100. There is no better off-road tire that is at least tolerable on pavement. And they are not too expensive.

And VDL means Vehicle Damage Likely.

Rick "who didn't run the AC while driving an average of 0.5 mph when descending Pritchett Canyon, and still got lousy gas mileage" Denney