Some LED current ratings assume some really favorable test conditions - such as being surrounded by air no warmer than 25 degrees Celsius and some decent thermal conduction from where the leads are mounted. Next, know your supply voltage. It should be well above the LED voltage for reliable, stable LED operation. Use at least 3 volts for the lower voltage types, 4.5 volts for the 3.4 volt types, and 6 volts for the 430 nM blue. The voltage in most cars is 14 volts while the alternator is successfully charging the battery.
Next step is to subtract the LED voltage from the supply voltage.
This gives you the voltage that must be dropped by the dropping resistor. Subtracting these gives 2.6 volts to be dropped by the dropping resistor. The next step is to divide the dropped voltage by the LED current to get the value of the dropping resistor. If you divide volts by amps, you get the resistor value in ohms. If you divide volts by milliamps, you get the resistor value in kilo-ohms or k.