The conventional wisdom is that it's more energy efficient to close the windows and use the A/C. And by "conventional wisdom" I mean I heard it on "Click and Clack, The Car Guys". It depends on your speed, of course, the faster you're driving the more aerodynamics come into play. In stop and go traffic, windows down is the most efficient - but you don't get any cooling.

You can also keep the windows up and just run the fan, which gives you some cooling with negligible fuel use.

I try to buy cars with higher than average gas mileage so I don't have to worry about sacrificing a few mpg to use the A/C. It's hot and humid here in Houston.

Thanks, I think this is correct for my case because it's all highway driving. I'll open the moonroof and all the windows sometimes just for fun, even tho it's costing me money, it's worth it.

I listen to Car Talk every week. They really make me laugh. I love them in Cars too.

I've been known to open the moon roof at night and drive with the AC on, as well.

Look at this Hongkong taxi: the double top is aerodynamically correct and prevents the "real" top being heated by the Sun.

Cunning!

Maybe you could have cardboard passengers...to stop the real passengers...

silly silly silly...smack my paws!

Depends on the car, speed, engine size vs aircon compressor size, etc. Several manufacturers have said that it is more efficient to put the AC on, and that is correct, in many cases. On the other hand, the Toyota Prius suffers a very large reduction in fuel efficiency when the AC is on (about a 25% reduction in mileage) largely because the power required to move the car down the road is low, so the AC consumption is relatively large factor.

The Myth Busters got different results in different tests.

In practical terms, it can be considered a toss up for an "average" car. For any individual vehicle, you can read the manual, and go with the manufacturer's recommendation (if there is one), or you can run tests both ways -- which is the only valid way to decide, for a particular car, particular driving style, particular speed, and particular temperature.

Keeping the AC at a reasonable temp helps a little, because the compressor will cycle less frequently. I'd guess that, in the vast majority of cars, with the AC set for the high temp side of comfortable, most of the time, it's better to put the AC on.

BTW, there is another CR4 thread on this subject from perhaps a year ago.

Hey..I know what you shouldn't do...

I'd been on a business trip with my boss, we got back into the car which had been parked in the sun and was really hot inside... I started opened the window.

He said 'no, I'll put on the air con'.... groan, bonkers!

So we sat in a stifling hot car for about 5 mins while the air con worked it's socks off to remove the heat. WRONG!

Open the window (assuming you are moving) to remove the hot air real quick.....then close it and put on the air con if you want to cool down some more.

Hi,

I have my champagne-coloured car at the body shop and am driving a black rental. I am amazed at how much hotter the black car is, and using the A/C is the only way to survive in it. I'm happy for the lesson, though - I'll never buy a black car, even if it does look better in the shade!

I work for a local municipality in the Phoenix area. You want heat? We've got plenty of it. All of our city trucks are painted white, and my personal truck is white, as well. Staying cool in the Arizona heat isn't easy, but as a previous submitter said, at least you can put on the fan, just to stir up the air, a bit. All of our work trucks have a/c, and it's a good thing too. One problem though...just try getting out of an almost frosty air conditioned truck, and right into the 110* heat, will damn near make you pass-out. We have had 3 weeks in a row of temperatures of 110* or higher.

Waterman

Yes, and dont forget to wear your driving gloves to open car doors or touch the steering wheel when driving in daylight hours in AZ.

Glad to see a fellow "Zonie" in the blog. The driving issue here is cool. With internal car temperatures approaching 300 degrees (dark vehicle, parked with windows closed) cooling quickly is paramount. My solution is roll down the window, start the car/truck (the A/C is always on) and roll up the window as soon as you feel cool air from the A/C. Mileage ... at 117F, who cares? The definition of Winter time here is when you run the heater in the morning, and the A/C in the afternoon!

From another "Zonie"...

Black cars also show dust more quickly and need washing more often. But, yeah, they LOOK cool...

For the long trips? Loose trunks. Zipper type sandwich bags. One ice chest. Ahhhhh.

Hey, it gets hot in TX

personally i run the ac with windows open. i like fresh air

Interesting, but probably qualifies as the LEAST efficient!

I'm worse than that. I run the A/C with the top down (or up if you're British). Do I get the prize for least efficient?

Do what is coolest and more comfortable to enter a good comfy state of mind while driving home. Your arms and legs don't know if you have money. Only your brain knows. So tell your brain at the end of your life you won't miss the 13 cents extra it cost to do this or that. Try this: Get out a $10 bill. Tear it up in little pieces. Did your life just change dramatically ? Nope. So stay cool man. This nickel dime stiff is lost in the noise!

Alternatively..

Take out a $10 send it to

Del the Cat, 10 Cat Street. Cat Town UK

Or your favourity charity... it will work on many levels.

Most efficient?

Please see #6.

In reply to 13, >to 12, "Re, #6" ... (no disregard for #15's common sense)... Hey, it depends on the time of year and where ya are.

I can tell you from first hand experience, it is Not ... repeat, *NOT* efficient whatsoever to cruise on down the road with a window cracked 1/4 of the way open, (saving on a/c consumption) and be crazily veering whilest swatting at some unidentified buzzing object that keeps swatting you on the side and back of the head, while you are looking for someplace to pull over and wait (all doors wide open) for it to leave of its own accord...!!!!

(Anybody tried that "Recharge-it-yourself" kit that I've seen advertised on TV recently? I DESPERATELY could use it, if it works!)

When you turn on the defroster in a typical car and the temperature is above 60 degrees F or so, you ARE TURNING ON THE AIR CONDITIONER, if your car has A/C, to help with the defrosting. If your heater control is turned to minimum (off), then you are getting A/C to your windshield.

roof top of vehicles are main culprits they absorb heat , if insulations are added ,like poly uthrane foam atleast cabin will not turn into a oven , foamed seates are also heat conservers , replacing such seates with cotton fabric , springs ,paper fabric will enhance the coolness of interiors , also replacing interior clumpsy plastic finishing with wooden one will be better if you can do so as such modifications are difficult to make , i have learnt some people in third world countries do such arrangements in there trucks , jeeps .

I don't know about the most economic method per your question, but there is another factor to be considered. If you do not use your air conditioner on a regular basis, the seals will become dried out and the system will leak out the refrigerant. The system contains a refrigerant which also works as a degreaser. There is also an oil in the system to keep the moving parts of the compressor and the seals lubricated. If this combination is not mixed around regularly then the seals deteriorate.

What is your speed? If your in traffic, I believe it would be a lot cheaper to open your windows, if your driving 65 and over it would be better to run your AC.

If you want to keep cool, have at hand a plant sprayer with water in it, spray your whole body with water and open the windows. The passing air will help keep you cool! alternativly you could rig up the squirters from the car as long as you only squirt yourself with water! It works for me!

This technique could be a dangerous distraction if applied to female passengers.

Every man is his own safety officer!

Unless they are driving and you are doing the squirting (of the spray bottle!)

It depends on where you are located, but if you are in a dry climate such as Arizona or New Mexico you can use a piece of equipment called a "swamper". You fit it at the top of your window with the window almost closed. On the outside of the car there is a tube with a large sponge and water reservoir in it and as you proceed down the road the wind goes through this evaporator and the water droplets come in the car and evaporate, thus cooling the interior.

The problem is that it won't work in a humid climate such as Florida or Georgia.

Close the windows. Turn off the AC. Meditate. Be in the moment with the warmth.

Ahhh. Reminds me of my days of Bikhram Yoga. motions at about 110F. No fans.

At one with my warmth

I had an idea for a product some years ago concerning this issue. Since it is unlikely that I will have the time to seriously pursue it, I will share the idea. You are collectively a very clever bunch of guys. Maybe you can make something of it. My career has been mostly in machining and toolmaking with as much product design as possible thrown in, but because my dad was in refrigeration, I spent a couple of semesters on the subject.

I don't have enough knowledge in the various areas of physics and electronics to determine if this is a viable concept or to pull it off if it is but, tell me what you think. If it is interesting enough we should open up a new thread and discuss it.

Now suppose you integrated into the roof or trunk hood of a car a solar panel with enough capacity to either charge a battery to run or directly run a very small compressor. The compressor would operate a small AC unit with the evaporator in the car possibly under the passenger seat and the condenser under the car in a protected placement. This unit would be not likely be able to totally cool off the car but it could be significant. It would only operate by thermostat and could be set on a timer so that energy would not be expended until just before it was needed.

What do you think.

Good idea but technology isn't there just yet. I just toured a science lab for solar cells (Sandia Labs in Albuquerque, 15 minutes from my home) and even with concentrators I'd guess the panels would be 2-3 roofs big and FOR SURE not cost affective even if 1 roof size would be available. maybe others can correct me on this as I am just guessing based on the super cells I saw versus output.

A kid on the "Inventor" show had his window hung unit that used solar to turn a fan past a coil of water to keep the car cool while parked. Not enough delta T, so he was bounced off crying with a reality check on how brutal investors can be. Also, there was a window seal gap and that wasn't cool for security. Keep thinking though!

The manual for my AC states: If the car was parked in the sun you must first drive for a distance to cool the air ways and piping slightly before switching on the AC.

If you don't it seems to take longer before the correct temp is reached.

In my SUV and the Mini Van I lways 'vent' the cabin for a bit by simply opening all windows about a 1/4 mile rather than trying to cool the heated air that has been trapped. And if it's a particularly long trip, then....well ya'll know what I do.

CR3

One thing I see people do all the time is NOT put the AC on max, thinking they are saving energy. But on a lot of cars MAX just means recirc. Newer cars seem to have changed and added a seperate recirc button. But the AC has to do less work to cool cabin air already cooled then the outside air. This is not recommended to keep on for all 4 hours of a long road trip as the air inside the car will get very dry also initially you may want to keep it off the first half mile or so as you are evacuating the heated air in the car with the windows down.

Shawn

Fascinating! So many variables, physical and physiological. So much wisdom, conventional and unconventional. How about this?

Find the answer to the question: "...how to 'keep' hot (driving the car) on a hot day?" Then, do the opposite. Should work for all possibilities.

Ok..I've extinguised the fire on the passenger seat and removed my overcoat and frogman's suit...what next?

I'll give you a picture of my ex-wife. That will chill you to the bones! You'll shake.

Before turning on the AC, I always start going for a few hundred meters with the windows opened "crossway" (that is, I open the driver's window and the passenger's side rear window; I have a four door car). It creates a real draft through the car (what feels like much more than if I openened all windows ?!?!?) and if I don't get jammed in traffic, I'm usually comfy enough to make it home without the AC.

Yes, I hear you-but you dont live in Houston, Texas like our fine fellow Bhankiii does. Trust me, I lived in Houston for 1 year, YOU NEED AC!

Hi James,

I realized that when I went home in the afternoon...I live in metric (for the most part) and read the temp in Celsius. It was 35 on the car thermometer which I thought was pretty hot. Then I switched it to Fahrenheit to compare and it only registered as 95 !!! I was thinking of retracting my comment but...

I agree completely: screw the savings and be comfy!

regards,

I, living in Texas, was interested in your crossways practice. So on the way home I tried it. I was not too impressed with that config as a whole, but it ws better than just front or all four. So I crossed the other way. Front passenger to rear driver. I found that it reduced unwanted gusts (i am a smoker -wheez, cough) and also road noise. while the breeze passed by trying to escape through the 1/3 -1/4 (not fully) opened rear window creating a nice cooling effect.

Thanks Rick.

Charles

You're welcome,

I think it might have to do with more of a "feeling" of being better or maybe the breeze going by providing more evaporation of the sweat and therefore cooling you off but at any rate, if it works (and it does for me at 95 F), then I'm glad I could be of service...

actually it depent on your priority:

1. if the priority just for energy, than it will better open the window.

2. if the priority is covortable driving, then it will better using AC on, because, no noise, dust get in your car and the temperature can be managed as cool as you want.

In the mornings when I leave for work and in the early evening going back home, I usually leave the A/C off and open the windows. I drive mostly at 60kph with occasional forays to 80kph. The time of day usually means that it's not that hot yet and I can rely on the air being cool.

If I'm driving between 9am and 5pm, I usually use the A/C full on. My cars an old '95 nissan and it's seen better days. I noticed that when I keep the thermostat at maximum (cooling, that is) the engine runs smoothly. If I set the thermostat lower, the engine revs up when the A/C cycles off. You can actually feel the sudden increase in power in the engine. In traffic, those high revs make me cringe. I can practically feel the $$$ draining out of my pocket (okay its pesos, not dollars ).

According to Mythbusters, when you're driving below 45mph (~70kph), it's more fuel efficient to drive with the A/C off and the windows open. Above 45mph, it's more efficient to drive with the A/C on and windows closed.

However, it's even more fuel efficient if you drive with the A/C off and the windows still closed . It won't do for comfort but you'll get farther.

Just for info. On a tankful of gas (I usually gas up when the indicator says 1/4 remaining), I get an additional 50kph or so if I don't use the A/C at all when going to/coming from work.

When it's raining, I forget about fuel economy and drive with the windows closed and the A/C on at about 1/4 to 1/3 on the thermostat setting. When I'm about a kilometer from my destination, I switch off the A/C. Reason being, when I step out of the cold car, my eyeglasses fog up .

Should you be very concerned about saving fuel, provided you drive below speeds in which aerodynamics is an issue: just have this chiller placed on the roof of your car.

It works making water circulate from a deposit trough an evaporator. A fan directs the air towards to your head.

I will have one installed in my Aston Martin next week!

You bought the car for your comfort.

If you can afford a car who cares about a few MPG loss?

When you park the car, keep windows cracked open a 1/8 or 1/4 inch on the opposite sides.

When you want to use the car, start the car, open all the windows or at least the front two and move a little.

Put the A/C on and drive on for a short distance.

When you feel cold air from the A/C, close all the windows and drive on.

Then put the nice soothing music and go home or to the office and do not worry about meager few MPG loss. Do not sacrifice your comfort over this tiny details and thank God that he gave you this comfort to drive and not use bicycle or Rikshaw or TUKTUK.

I do not know why people worry so much about the MPG and sacrifice their comfort and waste money on some thing else without any concern? Just enjoy your car.

Folks, pleas do not consider me a snob but I personally do not care about these minute detals and drive with comfort with A/C on all the time and specially in Houston, Texas and Shreveport, Louisiana where you can fry egg on the side walk.

I hope y'all have a very nice and blessed day.

Regards;

Nadeem Butt

08152007

You're entitled to your own opinion as everyone else is. If you're earning enough, sure, go ahead and be comfortable.

I don't earn a lot so fuel economy is a big thing with me. I don't use the A/C when I drive to work since it's still cool at that time. When I come back home since it's not hot anymore so I leave the A/C off. I'm comfortable and thrifty.

If I have to leave after 9am, I switch on the A/C unless it happens to be a cool day. All this thrift gives me an extra 20 to 30 kilometers for every 1000 pesos (about $20) worth of gas. Over several months that's substantial and I've had my car for six years now.

You are 100% right on all accord.

Every one has his or her own driving habits. There is no wrong or right way of driving. Just what you feel like doing is the right way.

Yes, by Grace of Gd, I make enough money to use A/C every time I get in the car and do not worry about the savings of petty cash. I do not remember when I did not use A/C except the winter days. Even in Anchorage,Alaska we ran A/C during June, July and August and whenever it went beyond 78 °F outside.

Also some times in winter, you should run / for a few minutes to make sure it runs when you need it next time. It was told by a sage mechanic long time agao and I do so in my car and at home as well.

You have a nice day.

Regards;

Nadeem