[btilly]

What's most striking to me as someone who has heard over and over again about how air resistance is a X^2 property, is the extent to which short (and presumably slow) trips punch above their weight in terms of range consumed per mile. From what I can tell, the actual cost here is the fixed cost of booting up some systems and electronics, and the variable costs of running them over fewer miles can make the MPGe drop. Starting and continuing to run the AC adds up!

Two factors.

First, if you go 10% faster, air resistance may be 21% higher, but you only take 10/11 for spending 10% more energy per mile. So higher speeds cost less than you'd naively think.

Second, at low speeds we start and stop a lot. Coming to a full stop requires actually putting physical brakes on and losing energy. The heavier your car, the more that this costs you. For slower traffic, if you look at distance traveled, number of full stops and energy, I bet that you can fit a linear model in 2 variables that fits the data better and gives you a sense of how much coming to a stop costs you.

[tlb]

That's incorrect. Power consumption is proportional to drag * speed, and drag is speed^2, so power is speed^3, and energy per mile is speed^2.

[D_Alex]

Or: Work = Force * Distance, so the energy spent per mile will be proportional to drag force, which is proportional to speed^2.

Either way, its 21% increase in energy consumed per mile in this case.

[btilly]

Ah, shoot. You are right. Energy is force times distance, momentum is force times time. I was thinking about force times time.

wipes egg off of face

[carlob]

True but we're interested in power consumption per distance traveled so it goes back to O(v^2).

[sokoloff]

That’s precisely what the last clause you’re responding to says. You’re agreeing but couching it as a “but”.

[modeless]

Starting and stopping does cost energy, but regen is pretty efficient and the lesser wind resistance of slow speeds mostly makes up for it. No, davidwhodge is right, the problem with short trips has to do with the fixed costs of "warming up" the car. Just looking at the energy graph in the car while driving it's easy to see that it's much more efficient after the car has warmed up.

[kube-system]

Yes, but electric motor regen usually does not work all the way down to zero mph. Usually it stops somewhere around 5-10mph, and the friction brakes take over. Slow rolling traffic that fluctuates between something like 10-30mph is very efficient, but stop and go traffic fluctuating between 0-20mph is pretty inefficient.

[ajuc]

Additionally if you drive 10% faster you spend ~9% less time on the trip which means heating/AC is working 9% less.

[baddox]

And there’s also some “inherent” value in spending less time traveling that is more difficult to quantify.

[londons_explore]

To balance with the inherent disadvantages of more speeding tickets and a higher risk of a collision.