What do these little green cars mean, inside the yellow bars?

If I remember correctly, it explains it on the display. Each car represents a certain amount of power that was provided by or into regen.

 

Regenerated energy.

 

In a gen 2, for every 50 Wh of energy regenerated during the five minutes covered by that bar, the bar earns one little car. 25 Wh earns half a little car.



Gen 3 also has the little cars, but they're only worth 30 Wh each. Inflation, I guess.

 

Psychology or statistics/math?

Perhaps the former so that drivers will see more little cars and get a "doing a good job" vibe.

Perhaps the latter because the sampling for the gen 2 is pretty coarse, and using a smaller unit provides a slightly more accurate representation. That logic can only go so far though - if a car was worth 5 Wh the statistical representation would be much better but there would be too many cars to fit under the bar. So they adjusted the value down, but not very far down.

I would have designed the display so that the bottom part of the vertical bar had a solid green section corresponding to the number of Wh with some appropriate scale factor so that it fits within the MPG bar. I guess it wouldn't really have to fit, if they inverted the top part could be green and the lower part yellow.

Is it possible on any Prius to have more little green cars than the height of the bar? I have never seen this myself.