Originally Posted by Hightower
...saved me researching the formular for changing the resirtance and altering the voltage
V=I x R therefore V ÷ I=R
Your voltage when the engine is running will be about 14v.
Your led specs said 30mA continuous running but they also said they derate as the temperature rises. Since neither of us know what the running temp will be, I reckoned about 20mA (0.02 amp) would be sensible.
So, 14 ÷ 0.02 = 700R. The nearest preferred value reistor is 680R. Voila.