Someone did the calculation that I've always wanted to see. The result is about what I would have guessed:
There are enough approximations in there that the final answer could be wrong by a factor of two in either direction, but the order of magnitude is absolutely correct.
Mandatory daytime running lights on all vehicles in the US would consume ~400 MILLION additional gallons of gasoline, add ~8 BILLION additional pounds of CO2 to the atmosphere and cost drivers ~600 MILLION dollars in extra fuel EVERY YEAR. Think about it, even if those numbers are twice the real value because of the approximations made. Scary, isn't it?