Does driving with the ac on waste gas?

Answer 1

At low speeds, AC uses more gas than driving with the windows open or closed. As speed increases, the additional drag from windows opened for ventilation uses more gas (drag increases rapidly with speed), and the break-even point comes around fifty mph. From that point upward, keeping the windows closed and using the AC actually saves gas.

Overall, however, the small saving is not worth the discomfort of driving in traffic while hot, regardless of the speed.