Gear B

The only thing it can hurt is your MPG, and then only if you forget to shift back to D after getting out of the mountains. It doesn't cause any meaningful extra wear on the 'transmission' or engine. While it does spin the engine up to higher speed as engine braking, the torque is significantly less than the regular propulsion torque. It will never over-speed the engine, the computers won't allow that to happen.

Its primary purpose is to prevent friction brake overheat on long descents after the traction battery has filled up from regeneration. Beyond that, use of B is a driver preference issue.

 

Unclear to me if on a downhill slope coasting the battery charges faster in D or B. In any case, if the slope is long enough that the battery will be fully charged before the bottom I turn on the headlights, AC, radio (not that this does much), and the defrosters to direct some of that energy away from the battery.

On a slightly related note, while it is true that the best highway MPG is generally obtained with cruise control on, its insistence on keeping the velocity very near the set point is suboptimal in some instances. In particular, rolling hills. Letting the velocity vary more (highest in troughs, lowest in peaks) would be more efficient. At least in theory. That said, it is tricky driving with as "light a foot" as the cruise control applies, so I am not confident that the MPG I would obtain by matching the optimal velocity profile would in practice be better than what the cruise control obtains.

 

what does the o/m say?

 

Nothing useful. It says that "B" is for engine braking, that there will be less engine braking than in other vehicles, and that's about it.

 

I've got a hill, not really long enough to justify B, but I've noticed: if I go down it in D the battery WILL be full, to the gills. If I shift to B at the top, I reliably end up with charge display a couple of pips below full.