[MikeSVX]

When your making a couple hundred horses it isn't really noticable, but in low horsepower cars you can feel it. Long ago I rented a Geo Metro with a/c in it. I was in California and I would have to turn off the a/c to get the thing to go up some of the mountains. Also, It is a common warning in desert areas to turn off your a/c when going uphill to prevent your car from overheating. This reduces the load on the engine and, the a/c heat exchanger is usually mounted in front of the radiator and will raise the temperature of the air before it passes to the radiator if the a/c is on.