In the page http://www.teslamotors.com/models if we scroll down we see the several flavors Tesla sells its Model S.
It puzzles me why the 85 version has a shorter range than the 85D. The first can run 265 miles, the second 270. Given that in the former the battery has to feed only one motor and in latter it has to feed two, why the first has a more limited ride distance?
It says right on the webpage:
Conventional all-wheel drive cars employ complex mechanical linkages to distribute power from a single engine to all four wheels. This sacrifices efficiency in favor of all weather traction. In contrast, each Model S motor is lighter, smaller and more efficient than its rear wheel drive counterpart, providing both improved range and faster acceleration.
This is pieced together from articles about this and the talk we had from a Tesla rep at the car leasing company I work for.
The improved range of the dual motor setup arises from the front and rear gearboxes (one per motor, nothing interlinking them) having different ratios. Electric motors have a rpm range in which they operate most efficiently, having the motors geared differently means that the car has 2 road speed ranges (1 for each motor) in which the motors are at peak efficiency, covering much more of the car's total speed range.
Also, each motor can be smaller which may be more efficient than a larger motor as a lot of the time only 1 motor will be propelling the car, though that will be offset to some degree by the increased frictional losses from having double the number of drivetrain components.
On a side note, I don't have any info but my guess is that the shorter geared motor has a 1-way clutch of some sort to prevent it from over speeding when the car is travelling at high speed.
I am not an expert but I read in an article somewhere when Tesla introduced the P85D that the dual-motor version is more efficient at putting power to the wheels since it can use all four wheels instead of just the rear wheels. This even offsets the increased weight of the dual-motor setup and translates to a 2% increase in range.
Think of how a high-end computer, like a server or hand-built gaming machine, can run just as cool and make less noise doing it with several small fans in the case rather than one big one.
The AWD Model S has two electric motors, one between each pair of wheels in front and back. The RWD Model S has just one, in the back, where the differential would be in a conventional car.
Now imagine if Tesla had designed the P85D to have performance equal to the original RWD Model S. Each motor would only have to produce half the power and torque of the original single motor. The smaller motors stay cooler, though, so they run more efficiently (heat is the enemy of electric motors, and makes them less efficient, unlike internal combustion engines). So the sum of the power requirements of both motors in the AWD Model S would be less than the requirement of the single motor in the RWD car.
Rather than making the AWD and RWD cars have equal performance, Tesla instead chose to make the AWD car more powerful. The result actually is still more efficient than the RWD car, as you've noted; the engineers sacrificed some of the potential greater efficiency of the dual motors in the name of performance, but not all of it.
