What's the "B" mode in a hybrid all about? Find out.

Dear Car Talk

Dear Car Talk | Jan 01, 2009

Dear Tom and Ray:

We recently bought a 2008 Prius, and just love it, love it, love it. But we're having a bit of a discussion about how to use the "B" on the gearshift. I say it's like a downshift, and you should use it only temporarily, while going down a steep hill or coming to a stop. You use it to save the brakes, then return to "D." My husband thinks it's fine to drive most of the time in the "B" mode, since it regenerates electricity and recharges the battery. Who's correct? And if "B" is the gear to use all the time, why didn't Toyota just call it "D" and forget the separate braking option? Thanks for your help!

-- Gayle

TOM: Gayle, you're 100 percent right, and your husband, unfortunately, has his head up his brake line.

RAY: When you shift into the "B" mode, you increase the drag created by the regenerative portion of the brakes. And while that extra friction does produce some electricity to help recharge the battery, the "B" mode's primary purpose is to simulate downshifting, or engine braking, when you're going down a long, steep hill.

TOM: The "B" mode's secondary purpose is to give guys something to fiddle with while they're driving. And after THAT comes creating more electricity to recharge the battery.

RAY: In any car, hybrid or not, when you go down a long, steep hill, you run the risk of overusing your brakes. That makes the brakes overheat, which leads to brake fade (when the brakes become less effective). Even worse, if the brakes overheat badly enough, the brake fluid can boil, and cause the brakes to fail completely. So it's always recommended that you downshift in those circumstances, rather than ride the brakes, to keep the car at a safe, reasonable speed.

TOM: But since the Prius has an unusual propulsion system and transmission, they've essentially "faked" a downshift mode for just that purpose.

RAY: So unless you're driving under conditions where you specifically need the equivalent of engine braking, just leave it in "D," Gayle. But try not to rub it in too much. Remember, husbands have extremely fragile egos.

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