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Originally Posted by jeffsquire
Mythbusters (discovery channel) did an episode on this. The consensus is that A/C saves gas versus open windows. BUT, they test drove Ford pickups not Boxsters. Still this may give you guidance.
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I have been researching this windows down vs windows up + AC, and so far there is no rule that applies universally. I've read report findings and I've done it myself on many vehicles. My sample size is very small, evening including the reports I read, and my route has not been consistent. Generalizing I would say that sports cars have very little difference, economy cars tend to burn more with windows up + AC, and trucks and SUV's do better with windows up + AC. All of my own tests were on highway only.
I've observed many car I've owned and many that I've had access to over the years. I started this while in college and have kept notes on it ever since
I'm inclided to think that sports have excess power at highway speeds and are so aerodynamically efficient that increased drag of open windows has an equal effect to AC use, but both have little effect. Econo cars usually have "lowish" drag coefficients and low excess power at highway speeds. The increase in drag of open windows has a less detrimental effect than the AC power loss, so running the AC and maintaining highway speed burns more gas. Trucks and SUVs have excess power and really high drag coefs, so the AC power loss is almost negligable compared to the increase in drag.
Cars Tested:
Owned: 91 vette; 95 Camaro Z28; 03 Acura RSX-s; 03 Boxster S; 04 Mini Cooper; 98 Corolla
Borrowed/Rented/Dealer Loners: 03 Cooper-S, 01 Neon, 02 PT Cruiser, 05 F150, 04 Chev Trailblazer, 99 Expedition, 97 Explorer, 99 & 03 Camry
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: Expedition, Taurus, Camry & Tacoma