Ok I know the standard response is that windows down is less economical than AC on and windows up. However I just watched a Leftpondian programme on Discovery called mythbusters which tested this theory.
First they used a computer with an airflow sensor in the air intake of an SUV and did 5 laps of a race circuit with windows up and AC off, then 5 laps with windows down and finally 5 laps with windows up and AC on full. The results were 11.7 mpg, 11.4 mpg and 11.7 mpg. So far so good.
The next test was to drain the tanks of 2 identical SUVs, put 5 gallons of fuel in, then drive at 50mph (I think) until they stopped. One had windows down, the other AC on. This time the windows down SUV went 15 laps further than the other (about 30 miles).
They claimed the difference between the two tests was due to the computer measuring airflow and calculating the mpg from it, rather than fitting proper metering.