It'll back drive the engine when you lift off of the gas, spinning it faster to dissipate energy. You'll get some regen but less than if you apply the brakes to get the same deceleration. In the long run this will reduce your mpg.
Firstly, unless you are descending a mountain pass, you should never be in B. Now, if you are not decelerating, none. If you lift off the throttle, the computers will simulate more 'engine braking' and may in fact, use the engine as an air pump. (You may be a hazard to other drivers as you slow rapidly with no brake lights. Don't be that guy) If you press the brakes above 24 MPH, you are using the engine as an air pump, no gasoline is added, but engine friction is helping slow the car. the harder you hit the brakes, the faster the engine will go. Since you are dissipating energy with the engine, instead of regenerating electricity via the motors, you are losing battery charge you could have saved, so MPG will go down. B under the D in the Drive gears? | PriusChat
Any time you brake even with just regen, you will lose mpg. "B" will brake using engine and regen. This would make the mpg go down even more. I would use "B" for emergency braking or going down a long hill and my HV battery is full (no additional regen could be done). Depending on your speed, putting your car in "B" would kick off the ICE if the ICE wasn't already running. That would hurt your mpg as well.
Robert...I know you posted your message in the Prius forum, not Plug in. But it's normal around here that PiP owners would come here and tell you how great PiP is.
The other benefit is it reduces the heat generated during charging of the traction battery. If driving through a mountainous area, "B" can reduce how much the traction battery heats on descents. But it works best if on the climb you avoid drawing on traction battery power by backing off the speed enough that the engine handles the climbing load. This is not always possible but 'heat is the enemy' of our NiMH traction batteries. FYI, just be happy that this time it is only the PiP owners coming here to tell you how great their cars are . . . there are others who really don't understand this is "PriusChat" and we're not going to run out and copy their mistakes. Bob Wilson
On a flat or climbing road with steady speed and steady throttle position: absolutely nothing. No rpm increase or mpg loss. The effects happen when changing speeds, or on downhills, or when stopped.
