Implementations of the present invention include systems, methods, and apparatus for improving the fuel efficiency (mpg/kpl) of a motor vehicle during those times when the vehicle air conditioning system is operating. Whenever the driver takes his foot off the gas, or the vehicle engine is otherwise caused to decelerate, the refrigerant compressor clutch engages, allowing the compressor to operate on previously-imparted vehicle waste energy (e.g., imparted by the engine, or by downhill travel.) When the refrigerant pressure reaches a pre-set maximum value, the clutch is deactivated, and the compressor stops. When the refrigerant pressure reaches a pre-set minimum level, the clutch is activated regardless of the existence of vehicle waste energy. When the refrigerant pressure reaches another pre-set level between the aforementioned maximum and minimum levels, in the absence of any vehicle waste energy, the clutch is again deactivated and the compressor stops.