> 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.

It depends. With conventional AWD (like all Subarus), you are correct that there is significant energy loss due to the AWD because there is a drivetrain to the rear wheels, even if it is not delivering power. Many hybrids, however, have AWD and they work by having a second, small electric motor that powers the rear wheels only when needed. That means no physical drivetrain to the rear wheels, so no energy loss. That's why a hybrid Toyota RAV4 AWD gets about 15% better gas mileage than the smaller and lighter hybrid Subaru Crosstrek AWD. The disadvantage of the electric motor approach is that it cannot deliver as much power to the rear wheels as a conventional physical AWD drivetrain can. That said, probably only an issue in deep mud or snow.

Well, no physical drivetrain to the front half of the car would make a difference. But even with electric motors in the rear there is a drivetrain between the wheel and the motor. If the idle rear electric motors are is still spinning then the total mass of spinning parts might well approach the mass of a Subaru drivetrain. Driveshafts are very light.

It sounds like the savings are more in construction costs than fuel savings.

It's more to do with avoiding drivetrain binding than fuel efficiency (although the two are related). When you corner, if all four wheels are driving at the same speed, then you'll end up forcing wheels to rotate faster or slower than they need to go. As a rule, when you corner, your front wheels travel further than the back wheels.

If your back wheels are rotating at the same speed as the front wheels while you turn, then you'll induce wheelspin on the rear and potentially lose control, or damage your tyres, drivetrain, or engine.

Full-time 4WD and AWD cars have three diffs, one on each axle, and one between the two.

Others have mentioned the RAV4 which actually has two separate drivetrains, which is a different implementation all together.

most AWD (mostly FWD) vehicle's engines are tranvserse mounted (horizontal in engine bay). These vehicles usually do not have a traditional transfer case/differential liek a 4wd vehicle, but instead a sort of "power siphon" that puts power to rear wheels selectively.

Most RWD and 4wd vehicles are longitudally mounted, allowing for easier packaging of power to the rear of the car.

The transverse packaging works much better in FWD configuration as

1. Packaging; the relation of the "AWD" (mostly FWD) transmission and a tranvserve engine is naturally a smaller form factor than other combos. Plus, the lack of traditional diff/transfer case is tons of weight savings.

2. Average driving; "FWD mode" in AWD vehicle is functionally equivilent to a FWD vehicle. This is ideal for most driving, IE; snow, fuel economy, understeer etc.