2/28    Stupid question, at what speed can I safely switch from D to 2 on
        my car? Thx.
        \_ Stupid answer: probably depends on your car.
        \_ At speed below which 2nd gear will bring your RPM to the red line.
           In addition, the lower the speed the better it is for your
           transmission.  If your car is 4-speed, you may be jumping from 4th
           to 2nd which is worse.  Also, your tail brake light doesn't lit when
           your car decelerates from shifting down, so you may get rear-ended
           if the driver behind you isn't paying attention.  Of course if
           you're trying to save yourself when your brakes fail while going
           down hill, shift by all means.
           \_ What "wears out" when I down shift to slow down on hills?
              Puts stress on the engine? Transmission? Breakpads are easy
              to replace, engine is not. I keep hearing car gurus recommending
              to use engine break, just wondering....
              \_ Transmission, but don't worry about it unless you're really
                 abusing it. The biggest problem is remembering to go back
                 to D after the hills.
              \_ shifting down w/ a big mismatch in speeds causes high
                 wear and strains on the transmission and clutch(es). steady
                 engine braking, e.g. on a hill after already in the right
                 gear causes much less wear but can overheat an automatic
                 transmission because more of the energy is being absorbed
                 (converted to heat) in the transmission's fluid couplings
                 than in a manual gearbox.
              \_ As for brake pads, the issue here is not how many months your
                 pads can last before they need replacement, but how many
                 minutes they can last before they fail before you reach the
                 bottom of the hill.  When brake pads (or discs/drums, not
                 sure) overheat, they fail all of a sudden, and you can't brake
                 anymore.
                 \_ If your car gets regular maintenance this shouldn't be an
                    issue unless you drive like a maniac or drive a really
                    shitty car.  Transmissions are a lot more expensive to
                    repair/replace than brake systems.