I took in account that I drive a brick, and big engines don't take such a big hit in MPG's from extra accesories because the amount of load put on it isn't very burdensome to the engine, whereas a small engine takes the burden more seriously.
So in the city, I drive with either vents on, or windows down.
On the highway, this is what I do. I don't see why more people don't do this, because it has shown me excellent results. Instead of running the a/c the whole time, I have the vents on, and when I get too hot, I press the A/C button, wait till the compressor has cycled twice, then turn off the a/c and use the vent again. The air remains cold for about 3 miles, and then goes back down to regular vent temps. I can usually stand it for about another mile, then i get hot (it's a good thing I'm cold all the time anyway
) then I let it cycle on twice, and turn it off.
My drive is about 10-12 miles, so I only have to press the a/c button three times. a whole lot better than driving 10 miles with the A/C constantly cycling on and off.
I judge when it cycles on by the volt gauge, because when the volts go down about 1-2 volts, (from about 14) that's when the compressor is running.
When on the highway, I never have the windows down. brick+open windows=giant brick parachute.