All-wheel-drive Tesla electric cars rated more efficient, but how?

2015 Tesla Model S 70D in new Ocean Blue color

2015 Tesla Model S 70D in new Ocean Blue color

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One of the myriad  advantages of electric propulsion, we’re told, is the efficiency of all-wheel drive.

The AWD versions of the Tesla Model S, for example, are rated as more efficient than the rear-wheel-drive versions of the car.

This flies in the face of 100 years of automotive history, during which vehicles with gasoline or diesel engines and all-wheel drive invariably paid an efficiency penalty.

DON'T MISS: Tesla Model X gets new 75D base version, replacing 70D that was never built

That was due to the extra weight and higher driveline friction inherent in conventional mechanical AWD systems.

When Tesla introduced its first dual-motor all-wheel-drive Model S electric car, in late 2014, the EPA efficiency ratings for the Model S 85D were 12 percent better than its single-motor predecessor, the S 85.

The 85D is rated at 100MPGe, compared to 89 MPGe for the 85. Measuring in kilowatt-hours used per 100 miles, the EPA says the 85D uses 34 kwh compared to the 85’s 38 kwh.

2015 Tesla Model S P85D - 'Chiseled by man and nature' [photo: George Parrott]

2015 Tesla Model S P85D - 'Chiseled by man and nature' [photo: George Parrott]

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By either metric, that’s a 12-percent efficiency advantage for the 85D.

When the dual-motor 70D was introduced in April 2015, it, too was rated more efficient by the EPA than the rear-wheel-drive S60 it replaced—this time, by 6 percent.

How the heck did Tesla do that?

CHECK OUT: Tesla Model S Dual-Motor Is Quicker, Has Higher Range Too: How Do They Do That? (Oct 2014)

“Because we have two drive units, we can shift the power from front to rear," Tesla Motors CEO Elon Musk explained at the time, "and constantly be at the optimal efficiency point for each motor."

"We’re actually able to overcome the penalty of the increased mass of the motor."

Tesla patent documents suggest that the rear motor is tuned and geared for acceleration, while the front motor is tuned and geared for efficient highway cruising.

2016 Tesla Model S P90D and Boeing 737 drag race

2016 Tesla Model S P90D and Boeing 737 drag race

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The car’s computer calls on whichever motor, or combination of motors, can deliver the required power most efficiently. The result: higher overall efficiency than a single-motor RWD car.

That’s the theory, anyway. And it’s a theory firmly backed up by the Teslas’ official EPA efficiency numbers.

But I’m sorry. I ain’t buying it. At least not all of it.

Real-world experience

In addition to 60,000 miles and more than three years behind the wheel of my 2013 Model S 85, I’ve spent time in both the 70D and the 90D (the successor to the earlier Model S 85D).

ALSO SEE: 2016 Tesla Model X electric SUV first drive by Model S owner

In warm weather, my single-motor S 85 typically registers  energy consumption of 290 to 300 Watt-hours per mile (Wh/mi) in my normal driving.

Since the 90D and 70D are claimed to be 12 to 13 percent more efficient than my car, I had expected them to show me readings in the 260 to 270 Wh/mi range.

But neither car came anywhere close to that.

During my 30-mile test drive of the 70D, I averaged 289 Wh/mile—virtually the same as the 291 Wh/mi my car registered during the 70-mile round-trip drive to the test site, under similar road conditions.

I later happened upon a 70D at the Tesla Supercharger in Moab, Utah. I asked the driver if I could check his lifetime efficiency number.

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