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More fuel efficient: A/C or open windows?

Does this sound like you?

You get into your blistering hot car that’s been baking on the macadam the entire July day. So the first thing you do is put down the windows. The second thing is blast the air conditioning. At some point after you drive off, you may put up the windows or turn off the a/c, but not just yet…not until it’s closer to bearable in your four-wheeled oven.

It’s a pretty sure bet that there is nothing even remotely fuel-efficient about this technique, but heck, sometimes a gal’s gotta do what a gal’s gotta do.

Which brings us to the harder question: Is it more fuel efficient to drive with windows up and air conditioning on? Or is smart money on skipping the a/c and using the cooling power of air blowing in the open windows?

The answer is yes.

Ambiguity aside, it really is all about how your air conditioner works and the vehicle’s air resistance or drag. Most cars today are built to be more aerodynamic, allowing the air to pass across the surface with less drag. Open the windows, however, and that drag is increased — dramatically, at higher speeds. Like opening a parachute.

In addition, drag increases exponentially at higher speeds: Doubling driving speed from 35mph to 70mph increases drag by four times.

The Society of Automotive Engineers (SAE) conducted a study using a full-sized V-8 SUV and full-sized V-8 sedan in a wind tunnel and on the desert. The wind tunnel simulated head and cross winds on mileage, while the desert setting factored in speed and temperature.

The SAE study showed that at higher speeds, having the windows down produced poorer fuel efficiency than using the air conditioner, with more significant results in the more aerodynamic sedan. The difference? In the sedan at 55mph, windows down decreased efficiency 20% or more, while windows up and air on was a decrease of about 10%. The SUV, in contrast, was just 8% less efficient with the windows down.

So, it’s a better bet to have the windows up and the air on when cruising at 50mph and faster.

But what about those short errands or commutes? Well, driving around town at slower speeds creates less drag, making it more efficient to do your shorts trips without the a/c and with windows down.

Another factor: At lower speeds, the engine is producing less power, so it has to work extra hard to power accessories like an air compressor. At higher speeds, the engine is already producing enough power to run the engine and additional equipment without as much additional strain.

Conclusion: Driving 40mph or less, cool your interior with the windows down. At 40mph and faster, the air conditioner alone delivers greater fuel efficiency.

Green Feature: To reduce fuel consumption, use both cooling methods strategically during these hot months.

Just not at the same time.

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