I've two blue LEDs which I will place into the bottom of my two cupholders (one each). They're 5 volt, 30mA LEDs and I'm having an issue recalling my high school physics.

I've got a circuit diagram with my two LEDs, taking up a sum total of 10v from the 12v tap in the cigarette lighter, so I need a resistor in the circuit that will knock out the remaining 2 volts. However, without the amperage of the cigarette lighter tap, I've no idea how to calculate the ohms with V=IR since I have no I (the amps).

Ideas?