Every gear, CV joint and bearing the driveline has to go through is going to cause a loss to friction.
This becomes even more pronounced the more efficient the rest of the car is. A car that needs 20hp to maintain 60mph and is losing 0.2hp to the driveline has a 1% loss. But a car with basically the same driveline parts that needs only 5hp to maintain 60mph now has a 4% driveline loss.
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