Are we talking about the 5mm LEDs? They run a nominal 3.6V, same as whites. You could assume they go up to 4V, but don't go any higher or you'll release the magic smoke.
Car voltage is normally said to be 13.8V. This may rise a little if the engine is running very high revs, or may drop a little during starting. It is the upper end that you want to design for, to make sure that you don't blow the LEDs. The lower end is fine as you don't run the risk of blowing the LEDs then, they just dim a little. And it only happens when your engine is off or when you're cranking it.
I suggest putting 3 LEDs in series with a 120R resistor to soak up the rest of the voltage. You could go as low as 80R, but that may be pushing it. You can, of course, put several banks of 3 LEDs if you want to increase the amount of light.
A slightly more complex way would be to use a LM317, LM78xx series, or similar voltage regulator. With these, you could put 2 LEDs in series with the regulator (the regulator usually needs a few extra volts over the load.) The regulator could be set up for 7.2V or 8V and it will provide this constant voltage to the LEDs so your lights won't dim even under cranking (unless your battery is dying), or blow up when you rev up.
