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When driving, is it more efficient (in terms of gas consumed) to use the air conditioner, or to drive with all of the windows down? While the air conditioner reduces efficiency through drag on the engine, driving with the windows down increases air drag on the car. I know this problem could be easily solved with a better knowledge of how both of these systems worked, but I can't seem to find hard numbers to support it.

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This depends on driving conditions, mainly speed and temperature.

  • AC fuel consumption increases with temperature
  • fuel consumption due to open windows increases with speed

The SAE found that AC is more efficient at highway speeds.

Modern cars incorporate tricks to reduce AC fuel consumption: the AC compressor is preferentially engaged when you're slowing down and disengaged when accelerating, for instance. Using an electric motor to power the compressor can reduce losses too.

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TL DR: Windows down w/o AC nets better fuel mileage.

There was an episode of Mythbusters which covered this subject. They found windows down without A/C will run significantly further than will a comparable vehicle with the A/C on and windows up. Here's a writeup from the Discovery site which talks about it:

With five gallons of gasoline in each tank, the MythBusters saddled up their SUVs and circled Northern California's Altamont Raceway at 45 miles (72 kilometers) per hour to see which one ran out of fuel first. Before starting their engines, Jamie cranked up the AC, Adam rolled down his windows, and the pair set off on their race to nowhere.

In the end, Jamie's air-conditioned SUV stopped first, while Adam's window-cooled car ran for 15 miles (24 kilometers) more. Since rolling down the windows sustained Adam's SUV for a significantly longer distance, the MythBusters busted the expert advice.

Here's the video segment of the episode.

Mind you, there are naysayers out there who didn't like how this was tested and the result.

All-in-all I didn't have any huge issues with the testing results from Mythbusters, mainly because I believe there was equal amounts of gas used in both vehicles and they were fairly evenly matched. Then, after the testing was complete, there's a huge difference in what each vehicle traveled. The difference is non-trivial: it is significant.

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    Mythbusters' testing was limited to low speeds (45/55 mph). In my car, driving with all 4 windows down, turbulence (and as a consequence, drag) rapidly rises from ok to very uncomfortable when accelerating through that speed range. So we might both be right: open windows use less fuels at low speeds, AC is more efficient at high speeds. – Hobbes Sep 7 '17 at 14:56
  • @Hobbes - Could be. The OP was asking for objective data, so I provided. – Pᴀᴜʟsᴛᴇʀ2 Sep 7 '17 at 18:49
  • interesting both SAE (above post) and mythbusters used big suv's / v8 sedans. Do the experiment with something highly fuel efficient and aerodynamic and I expect you will see far more sensitivity and so convincing results. – agentp Sep 8 '17 at 17:33
  • @agentp - You should roll out there and do that ... I'd definitely be interested in the results! – Pᴀᴜʟsᴛᴇʀ2 Sep 8 '17 at 18:57

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