In city driving acceleration is obviously affected when the A/C is on, and I'm sure it hurts fuel economy some but I don't drive regularly enough in the city to notice it. On the highway there's a little power loss, but I can't discern any difference in gas mileage with A/C on or off. A very narrow temp range (unless it's raining = no windows) I'll toggle the A/C on and off every few minutes, but otherwise I leave it on, temp. knob all the way to cold, and use only the fan to regulate the temperature - I've never needed to sustain more than speed 3. Defroster is a different story - I'll toggle it depending on conditions. Sometimes, if I anticipate needing to make an aggressive maneuver, I'll kick the air off ahead of time so I have all the juice from the get-go, but that's very rare.