The correct fan belt, properly installed on the loose side, worked well, while other belts, installed loose or tight, would break frequently, giving the engine fan and belt design an undeserved bad reputation.
The correct fan belt, properly installed on the loose side, worked well at 0-10,000 plus rpm. Any other belt, installed loose or tight, would rarely perform above a measly 2-3000-rpm, giving the engine fan and belt design an undeserved bad reputation.