Aerodynamically, the slower you drive, the more efficient you drive. The exact curve will depend on air pressure, amount of moisture in the air, etc.
For the engine, it is more complicated. You will want to keep its temperature and rpm at ideal levels. rpm and the gearing in the drive train determine what speed(s) that corresponds to (I guess drive train losses will depend a bit on the gearing, too, but that effect should be minimal)
For the two combined, this gets really complicated. Do you drive at the engine-optimal 45 or does the gain due to lower speed offset the loss in engine efficiency at 44? If 44 is more efficient now, will it stay that when it starts to rain? When you enter a different road surface? When your tires heat up and tire pressure increases slightly? When you have a window open?
http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Spe... shows how convoluted this can get.
http://www.fueleconomy.gov/Feg/bymodel/2012_Toyota_Prius.sht...
http://www.teslamotors.com/goelectric/efficiency
(the graph)
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.
http://www.teslamotors.com/blog/model-s-efficiency-and-range
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
+ 106.19966024840329
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:
It's a fixed gear, so this is actually plausible.
On an automatic transmission the transmission control module will figure out the gear for you at given conditions (won't let you get too low in the same gear) and what you say will be true.