What resistors to use with 3.8v LED's?

I have some 3.8v UV LED's, and I want to run them from my trucks 12v power supply does anyone know what resistors I should use to prevent them from burning up? Also I got a great deal on them @ $1.50 a piece, if anyone is looking for them.

LED Specs:

Emitting Color: UV
Wavelength(nm): 395
Lens Type: Water Clear
Typ. MCD: 140mcd
Typ Voltage: 3.8v (4.2v Max)
mA: 20
Viewing Angle: 12
Lead Length: 27mm

 

[DJ Scoto]

I found the answer for those of you that are intrested...

Use the following formula: Take the car voltage (Use 14v) and subtract the voltage of the LED (mine are 3.8v) Divide that number by the mA rating of your LED (mine are 20mA or .02A)
14v - 3.8v / .02A = 510 ohms. I chose 470 ohms. The LED voltage and amperage that I used in the formula were "typical" ratings. LED's will have a max rating also. (Mine are 4.5v, and 30mA) One way to test to see if you are providing too much juice to the LED is to feel the resistor and diode after a few minutes of use, if they are HOT there's too much juice, use a higher value resistor.
I found that 470 ohms is perfect for my LED's.