"and never have seen it spinning".. That sounds like it might have been installed in a bad location.
I found this..
How to Calculate Wind Loads From Wind Speeds | eHow.com
"Calculate the force exerted by the wind at your target speed by squaring the speed in miles per hour and multiplying the answer by .00256.
For a 30 mph wind the calculation would be
30 * 30 * .00256 = 2.304.
This is the amount of pressure, per square foot, a 30 mph wind exerts."
So, if we double the speed. 60 * 60 * .00256 = 9.216 per sq foot..
Instead of double the force (with x2 wind speed), we got almost 4 times the force..
One of the reasons you get much better MPG driving at 30 instead of 60..