New lights for my passenger cars.

[Madman]

Thank you , gentlemen. It's starting to penetrate.

 

[corgi]

I adopt the kiss principal and use the 20mA rule.
I always put a series resistor in cct with a LED and rather than looking up characteristics for each type of LED of which you more than likely will not have access to use the 20mA rule. That is we will only be supplying the LED with 20mA.
Therefore the resistor value R = (E(voltage) :- 20) x 1000.

for 3 volts = 150 ohms.

Now some may argue and it may be a bit dimmer, but it is the easy way.

If you want it a bit brighter use the 25mA rule. For 3 volts = 120 ohms.

Now aint that easy and you really dont need to be a wizard.