Picked up my P85D today!
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I still have my old 2013 P85, so I did a controlled 15 mile loop back-to-back to compare efficiency.
Same route, same time of day, same speeds, mostly 65 mph, same temperature. Only differences should be tires (21" Continentals on the P85, 21" staggered Michelins on the P85D) and the dual motor itself. Note, there is considerable elevation gain (and loss) in this loop, hence the higher than normal averages across the board.
Here are the results:
P85, 21" Continentals, 50F, 15.5 mile loop (65mph)
5.1kWh used for an average of 329Wh/mi
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P85D, 21" Michelins, 50F, same 15.5 mile loop (65mph)
5.4kWh used for an average of 350Wh/mi
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So, in this test the P85D used 6% more energy.
I think it's fair to say that when Elon said the dual motor increased range, he was referring to the standard 85D.