Car question: open windows vs. air conditioning

I have a 2009 Camry Hybrid. It has 2 levels of AC/heating: regular and eco, which works at a lower energy level. I’ve been reading that there’s a lot of drag when you drive with the windows open, lowering fuel efficiency. What I don’t know is which one is actually more efficient–the open windows or the air on eco or even regular AC. I usually keep the temperature high–just enough to cool the inside without it getting too cold–around 79 degrees. Any ideas?

Depends on the car and how fast you’re going, but for an efficient a/c traveling at highway speed, Cecil seems to think it’s better with windows closed:

I’d say windows closed, AC on economy, especially with a hybrid specifically designed with a low drag coefficient.

Okay, related question: on a hot day, do you open the windows after you start the car to “push all the hot air out”, or do you run it on the Recirculating setting, thus eventually cooling all the hot air which is inside the vehicle. My mom always insists that she lower the windows; I just let the air recirculate and cool. Which is better?

I take the T-tops off, turn up the Def Leppard and floor it.

This is one of those things that really varies from car to car, and on how you do it.

I used to drive with just the rear windows down, with the front windows slightly cracked to avoid reverb, and this consistently got me better mileage than using the A/C. And this was driving on the freeway to work, 30+ miles one way.

Try it with your vehicle, but remember that variations in driving patterns, etc can play a factor. I would definitely test each method for a full tank of gas before making a determination.

Er, I noticed you have a Hybrid, so I am not sure what your overall range is, etc, so this might not work for you the same way it worked for me.

Thanks Bill. That’s what I was thinking, especially since there’s a special setting, but I couldn’t tell.

This is another great idea, but I think it might ruin my roof, since I don’t have T-tops. I’m not so sure about Def Leppard either. :slight_smile:

If you can find the episode of Mythbusters where they tested both theories, it might help you decide what to do for yourself. I don’t really remember what they concluded.

When you get into the car on a hot day, the air inside the car will be waaaaaaay hotter than the air outside. So, it stands to reason that running the A/C on recirc will result in warmer air blowing on you than if the A/C is sucking in from outside - only once the inside temp is at or below ambient does recirculation get you anything. Also, you can displace most of the superhot inside air very quickly by running with the windows down for a couple minutes, usually bringing the inside temperature down to the ambient level much quicker than just running the A/C with the windows up.

But in the end, the correct answer to this question is “whatever makes you feel the most comfortable.”

It depends heavily on your speed, at lower speeds there’s not much drag.

Here’s a summary and criticism of that episode:

Of course in reality it would depend on many factors. I suspect speed is the most important factor, as already pointed out. It would also depend on the engine size, how aerodynamic the car is, how hard the AC needs to work (which in turn depends on how strong is the sunlight, what color is the roof, was the car hot to begin with, etc), etc, etc.

Automotive air conditioning is a power hog. Cars are poorly insulated, have high solar loading (both courtesy of the large window area) and insane convective heating (think “convection oven with a 70-MPH airflow”). There’s a photo out there on the interwebs called “Redneck Car Air Conditioner” featuring a car with a generator bolted to the trunk lid and a plug in room-A/C unit stuck in the car window, but the reality is that this would barely begin to cool the car. Real automotive A/C units draw something like 10-15 horsepower. Typically the compressor runs intermittently so that the time-averaged power draw is considerably less. But your car only requires about that much power to cruise down the highway, so this can dramatically impact your fuel economy.

Casual fuel economy tests conducted in uncontrolled conditions are notoriously unreliable, as there are a great many variables that can affect performance: wind, road grade, speed, acceleration habits, tire inflation, temperature, humidity, traffic jams, and so on. For these reasions Cecil’s test results are not particularly meaningful here, and even the response (from the folks at the Florida Solar Energy Center) doesn’t describe enough detail about their test conditions.

I work in a lab where (among other things) vehicles are tested on chassis dynamometers for MPG. For these tests, the dyno is calibrated to simulate the inertia and wind resistance of the test vehicle on a flat grade with zero ambient wind; the tires are inflated to a specified pressure with a calibrated tire pressure gauge; and the engine is fed air of a fixed temperature and humidity. The technician is required to drive a very specific speed profile during the course of the test (different profiles for highway and city mpg tests); he is allowed very small margins of error, and if he falls outside those bands, the test gets repeated. Even under these carefully controlled conditions, MPG may vary from test to test (for the same vehicle) by as much as Cecil was reporting.

For a more thorough (and thoroughly documented) analysis, consider this paper from the National Renewable Energy Lab, published in 2000.

Vehicle air-conditioning can significantly impact fuel economy and tailpipe emissions of conventional and hybrid electric vehicles (HEV) and reduce electric vehicle (EV) range. In addition, a new U. S. emissions procedure, called the Supplemental Federal Test Procedure (SFTP), has provided the motivation for reducing the size of vehicle air-conditioning systems in the United States. The SFTP will measure tailpipe emissions with the air-conditioning system operating. Current air-conditioning systems can reduce the fuel economy of high fuel-economy vehicles by about 50% and reduce the fuel economy of today’s mid-sized vehicles by more than 20% while increasing NOx by nearly 80% and CO by 70%.

How is the AC compressor powered in a hybrid car? If it is belt driven then the engine will have to work constantly thus reducing the mileage dramatically.

The Wikipedia article suggests it is electrically driven.

Thanks for this link. According to the comments, it’s better to keep the windows down at slower speeds and up at faster ones, because there’s little appreciable drag at slow speeds. One commenter uses 45-50 mph as his crossover speed.

Joe, thanks for the interesting information and the link that I’m going to now wade through.