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Drive vs B

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Posted by Max Yaffe on October 8, 2005, 3:28 am
I'm curious about the B drive setting in my 04 Prius.  How does the B
setting affect gas milage?  Is more energy returned to the battery in
B?  Or is D better?  When should I use B?


Posted by richard schumacher on October 8, 2005, 4:49 am

B enables engine braking and returns less energy to the battery, and so
it reduces fuel economy.  The intended use is while going down long
hills, to avoid overusing the friction brakes, just as one would put a
conventional automatic transmission into 2 or 1.  (*Unlike* a
conventional transmission, B does not give more low-speed torque.)  Some
people like to use "B" in heavy stop-and-go driving so they don't have
to touch the brakes so much, but again this reduces fuel economy.

Posted by Doug on October 8, 2005, 2:05 pm
Max Yaffe wrote:

Never unless you live on a mountain.

Posted by Earle Jones on October 8, 2005, 9:40 pm

Bad answer.

Use B whenever you would downshift in a car with conventional
transmission.  Using B will save your brakes on downhill runs,
either short or long.


Posted by Michael Pardee on October 11, 2005, 2:29 am
Since engine braking only adds a modest amount of drag at cruising speeds
(which is the same thing regenerative braking does, although regen can
provide more drag), it really doesn't take any load off the friction brakes
unless the battery is "full." In our 2002, that apparently is after
somewhere between 1000 ft and 2000 ft of descent. This seems to be borne out
by people who have reported checking their wheels after fairly long descents
and found they were cool. For downgrades that drop you less than 1000 ft,
"B" probably isn't useful.


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