recently i did some fuel consumption calculations, they are available here:
electronics engineering notes: How much can DRL lights improve fuel consumption?
HypermilerAX: I see that you've forgotten about engine efficiency. In my calculations I used peek engine efficiency but in real life you can expect much lower efficiency.
astra74: I was thinking about trying to sell this circuit but I'm not sure if it's good enough. Right now I'm introducing some improvements to circuit but maybe one day, who knows...