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Just as a radiator must be sized for worst case scenarios, the water pump, alternator, AC, and other components are over-sized for typical use. They are sized to deliver adequate performance under severe conditions that are not often encountered.
Just as grill blocking may be appropriate for some applications, under-driving various accessory and cooling systems may be appropriate. The gains from under-driving are miniscule to be sure. The overall load of the alternator, for example, still has to meet the demanded draw. While it may not output as much power when under-driven, it will make up the power difference by running longer. The same can be said of the AC.
The fuel savings really just come down to slightly less frictional loss due to spinning all accessories at a slower rate. In an efficient system with good bearings, this represents a very small gain. There are also tiny gains in fuel efficiency by having lighter weight pulleys that spin up to slower maximum speeds; less energy is wasted in accelerating the rotational mass.
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