Memes

Could have to do with efficiency at various motor speeds. With IC engines, they don't take constant fuel input and don't produce constant power or force over the rates at which they support spinning.

I understand electric motors have full access to their peak torque at any speed but I am not sure if that also means equal efficiency for the full rpm curve, so that could be a part of it.

Also I wonder about wheel efficiency and how that curve would look. Like if your tires are a bit under pressure, I could see more deformation being possible at lower speeds, which could mean the tires themselves become stiffer and more efficient as they spin faster.

And finally, 10 and 20 mph are both pretty low speeds as far as air resistance is concerned. Air resistance is more significant in how it scales. I forget the exact formula but I do remember that air resistance has a term with a higher exponent than the formula for work done (which boils down to distance travelled), which absolutely means that the force required to overcome air resistance increases much faster than the formula for the result of applying that force, but exponents can take a while go really get going.

Oh actually one more thing, when running a device like that, there will be some portion of energy it requires for just being on and another required for gaining and maintaining speed. It could be that the constant power requirements are significant compared to the amount of power required to get to and hold either 10 or 20 mph. Could even be a short or malfunction of some sort. If that's the case, then the fact that your 20 mph trip only took about half the time could have saved more power than the extra needed to move twice as fast.

So my guess is the amount of extra air resistance going from 10 to 20 mph was less than some set of benefits you gained from that same speed increase, of which I listed the ones I could think of above.