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 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. :)
Personally, I'd wager they followed the old scientific adage: "If you wish your function to be linear, sample two and only two points".
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
So ... no division by zero. BTW I worked this up with the help of Sage, my current favorite, free math tool: