Choosing a transistor and resistor for PIC input pin protection from Car Power
If you are using the NPN method as I described in #4 of my answer https://electronics.stackexchange.com/a/47250/4512, then you don't need a zener diode.  The B-E diode of the transistor will limit the voltage by itself.

To clarify, here is how you can use two resistors and a zener diode to safely drive a 5 V PIC input from car power:



With 12 V in, the resistor dividers by themselves would drop the output to 6 V.  However, the zener will clamp that to a bit below 5 V, which keeps the voltage safe for the PIC.  Even if IN momentarily went to 50 V, for example, the zener will limit the voltage at OUT to a safe level.

The transistor method looks like this:



The B-E junction of the transistor will limit the voltage accross it to around 700 mV, so there is no need to protect the resistor.  In this case I set the voltage divider so that it takes a few volts on IN to turn on the transistor.  If the base is about 600 mV when the transistor first turns on, then there needs to be about 5 V on IN for that to happen.  This gives you better noise immunity than just connecting a resistor to the base of a transistor without a pulldown there.

When IN is high, the transistor turns on, pulling OUT low.  Since the transistor can only pull down or not pull at all, you need something to pull the line high when the transistor is off.  This can either be the deliberate R3 as shown, or it can be a pullup resistor built into the PIC.  In the latter case, you don't need R3 and you just tie the collector directly to the PIC input without any additional parts.