A/C ,Speed,Gas....What speed...
 

What speed should you turn your A/C on ? I heard that if you open your windows at highway speed it doesn't help your MPG,and you should your A/C.But at lower speed open the windows to save gas.Damn 2 bucks gallon.

I've always used my AC regardless of fuel prices. I've not ever noticed a signifigant enough of a difference to make it worth sweating my ass off to save a few dollars at the pump.

I can't imagine it's that much of an impact. I usually use A/C anytime over 40 for a long (>30 minutes) drive. This isn't for mileage reasons, it's just that I can't hear the radio above 40. On a nice day, I just like to have the windows open instead of being couped up in the car.

What kind of vehicle? It's not likely you'll notice much difference unless you have an ancient car & a/c system. Old a/c pumps spun all the time or spun lots of mass even with the clutch disengaged. People often went so far as to remove the drive belt during winter months just to recover the 5-15HP. Modern pumps aren't bad. It'll make a difference, but it's much more effective to discipline your right foot and keep everything in tune. (plugs/ignition/O2 sensors/egr/pcv/etc)

i've got a 99 cougar, and i can really notice the difference in mpg in the city. i go from around 26 to around 22. on the highway though, the difference isnt that noticable. the highway i get around 36 with the a/c off and 33 with it on.

It all depends on the vehicle....my friend had a dodge neon that when you used the ac it had horrendous gas mileage, on my cadillac I haven't noticed a difference. The best way is to experiment and find yourself the right combination.