Awesome explanation, thanks @GerryAZ! Hadn't thought about it that way, makes total sense.
Dug into this a bit, out of curiosity...
Since drivetrain inefficiencies and rolling resistance are linearly related to speed, total resistance is dominated by air resistance at even moderate speeds, because wind resistance varies as the cube of the speed. So aero power (power needed to overcome wind resistance) at 100 mph is roughly 3 times the power requirement at 70 mph!
For the Nissan Leaf, aero power at 70 mph in a wind tunnel was 13.4 kw and at 100 mph was 40 kw, according to this source:https://www.tesla.com/sites/default/fil ... e-road.pdf
So, maintaining a speed of 70 mph, down a steep hill, while in B or ECO mode (i.e. using more regen) would reduce aero power by 26.6 kw versus coasting in neutral while maintaining an average speed of 100 mph.
If I've understood the physics properly, it seems clear that at higher downhill speeds it makes more sense to reduce speed and regen than it does to coast in neutral.
When traveling at lower speeds, I think it still makes sense to occasionally coast in neutral because reducing drive-train inefficiency will have a greater impact on reducing total resistance at those speeds.
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