> There are plenty of AWDs that operate as FWD 90% of the time, only engaging the rear as needed. (I am told this is for fuel efficiency but the logic escapes me.)
AIUI, you lose energy because the power/torque has to go through an extra differential, transfer case, etc, before reaching the rear wheels.
Going from the engine through the transmission to the front wheels is already a 'fixed cost', but the extra mechanics (and their friction loses) to the rear wheels an 'OpEx' that you don't necessarily have to pay if you don't need to.
>> the extra mechanics (and their friction loses) to the rear wheels an 'OpEx' that you don't necessarily have to pay
True for a FWD v RWD car decisions, but in an AWD are not the rear differential and driveshaft still turning whenever the rear wheels are turning? Even the clutch packs should still be rotating even if disengaged. The fact that there is no power through them doesn't mean they aren't generating losses.
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