fan

The original 7 blade fan does a good job keeping the engine cool but it takes away horsepower. The clutch fan does spin all the time. The clutch only engages when the radiator temperature is high at which time it spins at the same speed as the engine. When not engaged it spins approximately half the speed (RPMs) of the engine.
Both do the job but the clutch fan takes less power therefore helps the mileage.