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Originally Posted by curtis73
On the average, A/C for larger cars takes as much as 15 hp to run. That means you have to put that much more "foot" into it to maintain speed or to accelerate with the same force you're used to.
Overdrive does drop RPM, but lower RPMs aren't necessarily always better. Different engines have different peak cruise efficiency points. An overdrive on an already high-geared vehicle will cause the engine to lug down requiring more foot to maintain speed. For the most part, the manufacturers do a relatively decent job of finding a good cruise RPM, but sometimes they go a touch on the conservative side (RPMs too low)
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15 hp is a bit much, but up to 10 hp is possible. Mean power consumption is however much lower, perhaps a hp or two. With modern cars you also won't need to give more gas to compensate for this loss, that is done automaticly with the modern electronic throttles. The loss is however still present.
Older cars usually used to disconnect the AC compressor during high load (like acceleration), today this is usually solved by letting the engine produce a few more horsepowers instead.
A gasoline engine is more efficient at high loads (with the throttle more open), so a reduction in engine speed and an increase in produced torque (higher load) almost always result in a decreased fuel consumption. Even if it results in engine vibrations. The highest efficiency are usually found at about 70% of WOT, slightly less with a turbocharged engine. Load has a much larger effect on engine efficiency and specific fuel consumption than engine speed.