However, a continuously variable transmission allows the driver (or the automobile computer) to effectively select the optimal gear ratio required for any desired speed or power. The transmission is not limited to a fixed set of gears. This lack of constraint frees the engine to operate at its optimal brake-specific fuel consumption. An HSD vehicle will typically run the engine at its optimal efficiency whenever power is needed to charge batteries or accelerate the car, shutting down the engine entirely when less power is required.