Right, let's look into this.
1 watt= 0.001341022 horsepower.

Let's say that your headlights are 150W each. Hell make 'em 200W, so it's 400W total. (They're nowhere near that high, but let's run with this.)

400W= 0.5364088 HP. Even if you're driving a weedeater on wheels, that's what, maybe 0.5% of your car's output? Assuming that you get 30 miles per gallon (pretty common in the US) that's 0.15 mpg difference. You reduce your efficiency by far more than that by opening your window on a nice day.

tl;dr: twaddle.
( The Resident Loon Not a demographic. Do not measure., Wed 10 Oct 2012, 19:20, 2 replies)

That is for a 100% efficient alternator/ battery, and 0.5% of max horsepower, not average power. Not sure what the real numbers are. I'd guess around 30%, and average tractive power of around 8kW, in which case the numbers look a little different.

Anyway, I've seen figures that suggest moving from incandescent to LED DRLs can give you an extra 0.5mpg. Not the end of the world, but still a waste of fuel.
(silene, Wed 10 Oct 2012, 21:30, closed)

Chapter 4 of this: ec.europa.eu/transport/roadsafety_library/consultations/drl_20060727/drl_trl.pdf goes into a lot of detail, but suggest that average increases in fuel consumption are between 1% and 2% (depending on car) for a 160W DRL system.

Like I said, not the end of the world, but still significant over a years worth of driving. As far as I know, this is why incandescent type systems will not be allowed when they become mandatory over here.
(silene, Wed 10 Oct 2012, 22:39, closed)

I never realised that car lights could be so fascinating

( 2 Can Chunder Word to your mums, I came to prod bums, Thu 11 Oct 2012, 2:14, closed)