Because this amused me
I did a quick look into it.

Let's assume that you're driving a car with a teeny little engine- say, 90 hp. That translates to 67,113 watts. Just for argument's sake, let's assume that you're only using 10 hp at cruising speed, which is about 7457 watts.

Now let's assume that your headlights are 60 watts each (no idea how much they really are, but that's probably pretty close). Let's assume that the rest of the little lights add up to another 80 watts, just for the sake of even numbers.

200/7457= 0.02682043, or 2.68% of your engine's output at expressway speeds, where you're only using 10 hp. Assuming 40 mpg, that's a difference of 1 mpg, lights on vs lights off.

Negligible. And if your car is more powerful, it's even less.
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 16:31, Reply)