Too much underdrive for a race car is not much of a concern due to the high RPMs they run at, but for daily driven vehicles it can lead to a dead battery if too much time is spent at idle or low RPM.
Too little underdrive for a racing vehicle is not much of an issue due to the high RPMs they operate at, but for daily-driven vehicles it can cause a dead battery if too much time is spent idling or running at low RPM.