RomanV wrote:The mythbusters segment on it isnt really right.
The efficiency difference depends completely on how aerodynamic your car is to start with.
If your car is like a big giant brick, then the percentage of difference that winding the windows down creates is small, where as it makes a higher percentage of difference on an aerodynamic car.
Hmmm not really, it's still the absolute change in drag that's important, regardless of how aerodynamic the car was to begin with. If you increase drag by say 1000N at 100km/h, then regardless of the total drag, you're going to have to generate the same amount of extra torque to overcome that drag to maintain a steady speed. I would think, however, that a nicely streamlined car is probably going to see greater flow disruptions from having the windows down than a dirty great brick which is already surrounded in shitty, turbulent air.
A rediculously easy way to tell, is to have a multimeter monitoring your injector pulse width.
If it's higher doing one thing or the other at the same speed, then there's your answer.
Yep, and even better, if you have a car with OBDII and stick a good scanner on there, you can view real-time fuel use, which will quickly tell you which is the more efficient.
Personally, I don't really like the feeling of conditioned air, and generally go for windows down unless the noise or wind is too disruptive. It helps that I have probably only owned one car where the AC worked properly anyway.
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