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I'm not sure. The y-axis on this graph is just the rate of power consumption. It doesn't do much for you to use minimal energy while traveling 0 mph.

Efficiency, in terms on miles/gallon, is always a trade-off between traveling fast enough that the overhead of running the vehicle is less significant but slow enough that wind resistance doesn't dominate. There's a sweet spot in there.




The y-axis has units of [energy]/[distance], I think I'm interpreting it correctly? It seems to take everything into account in there.

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Yeah, I'm definitely confused. I don't understand how a car traveling 0 mph can have a finite value on that graph if the y-axis is [energy]/[distance]. I was hoping the units were [Watts][hours]/[minutes] or something to make everything better.

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I found they have another graph with the same units (written "Wh/mile", so that's unambigious). But it has a different shape, with a minimum at 20-25 mph...

http://www.teslamotors.com/blog/model-s-efficiency-and-range

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That graph shows a maximum efficiency at around 22 MPH (for the 85 kWh battery), which makes perfect sense -- any faster and air resistance becomes a factor. Any slower and energy uses apart from propulsion begin to eat away at efficiency -- the computer, lights, air conditioning, heat, and so forth.

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Much better. Thank you!

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> Yeah, I'm definitely confused. I don't understand how a car traveling 0 mph can have a finite value on that graph if the y-axis is [energy]/[distance].

The car expends energy just sitting at zero MPH -- the computer and lights, air conditioning and heating, things like that. So the graph is correct -- there is an energy expenditure to "go" zero miles per hour.

> I was hoping the units were [Watts][hours]/[minutes] or something to make everything better.

When the speed is zero, it stops mattering which units it's expressed in. :)

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When speed is zero, distance is zero and energy/distance is infinite. Hence the OP questioning how could the graph present a finite value at that point.

Personally, I'd wager they followed the old scientific adage: "If you wish your function to be linear, sample two and only two points".

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> When speed is zero, distance is zero and energy/distance is infinite.

1. Not energy/distance, but energy/speed.

2. Notwithstanding the cart's labeling ("Wh/mi"), its values aren't predicated on a division of energy by speed. "Wh/mi" doesn't literally mean mean "watt-hours divided by miles per hour", it means "the relationship between watt-hours and vehicle miles per hour".

> Personally, I'd wager they followed the old scientific adage: "If you wish your function to be linear, sample two and only two points".

Yes, except the chart isn't linear. Just for fun, here's a polynomial function that matches the chart's results reasonably well:

    f(x) = 0.03165008148658699 * x^2
         + 0.2559872636164877 * x
         + 106.19966024840329
Here's the resulting chart:

http://i.imgur.com/rLmaN90.png

So ... no division by zero. BTW I worked this up with the help of Sage, my current favorite, free math tool:

http://www.sagemath.org/

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