What value resistor to use to not blow up LED?

I've got an LED that draws about 40ma at 3.5V.
If I want to run it off of a 12V-14V source (my car), what value resistor should I use to keep my LED from fizzling out on me?

My own (and probably flawed) calculations look like 250-350 ohms should do it but I want to make sure I didn't look at things the wrong way.

Thanks in advance!
Jason

470 ohms will take it to 3v.

Use this; http://www.csgnetwork.com/ledsingresistancecalc.html

212.5 ohms is what you need for 12v, 262.5 for 14v. If you want to be on the safe side, get a 300ohm resistor or something.

And for future reference, when calculating resistance for LEDs:

Power Source (PSU) volatge minus the LED voltage (LV), divided by the current (A).

(PSU-LV)/A

For your specific situation....

(12-3.5)/.040 = 212.5 ohm

or

(14-3.5)/.040 = 262.5 ohm

just fyi, the calculator link above gives the same results.

While in school I learned that if you use a 1.2K-ohm resistor on 9 -20 v the led did fine. I know this is not as fancy (scientific) as use X for y volt and A for N volt but it works. Heaven forbid you step on the accelerator in you car and the voltage jumps to 15v your calculation would be all wrong.