I submit that it would take
25000mi of nighttime driving for each 6.7W #3157 taillight bulb to burn a gallon of gas at 50% alternator efficiency and 25% ICE cycle efficiency.
Will the bulb pay for itself? Eh, maybe a wash if you're lucky.
The big pros
and cons of LED taillights for me are the quantity, intensity, and directionality of their light.