Try this link
http://www.straightdope.com/classics/a2_393.html for an experiment that showed that at highway speeds, neither made much difference but the A/C did reduce the fuel mileage slightly less than having the windows down. On the other hand, another "study" indicated that at 45 mph the A/C used more fuel than having the windows down
http://msgboard.snopes.com/cgi-bin/u...0;t=000565;p=0.
The higher the speed, the more turbulence and wind drag in general so the less difference the impact of driving with the A/C on and the greater the impact of messing up the aerodynamics of the vehicle by having the windows down. The "general wisdom" is at low speeds the A/C uses more fuel but as the speed increases the numbers change and the windows down approach is more harmful to fuel economy
http://www.edmunds.com/reviews/list/...4/article.html.
Of course, slowing down on the highway and driving more conservatively in town will affect your fuel economy more than the A/C being off or on will.
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2003 Almond GT AutoStick, body colored rear splash guards and hood struts, chrome gear shift, AC & vent knobs, billet steering wheel spokes and pedals, AMX1397 Turbo-Intake Pipe.