Your maximum input voltage should not exceed Vdd, however a small tollerance (say 5-10%) won't harm.

See also the answer I just posted on "How to use VRef". Adjust your Resistor values accordingly so your maximum vehicular voltage (about 15v on an average modern vehicle give or take) never exceeds 5v when scaled down. A 3:1 reduction ratio would be about right.

Zeners are cheap (only a few cents) use in series with say a 10K Resistor. Don't forget to stick say a 100nF across the Zener as they're noisy devices.

You can also use a transistor as a buffer... but you're adding that and couple of Resistors so you're increasing your parts count.

Opto isolators are overkill especially if you don't need actual isolation.

Melanie