Modern car alternators provide power as needed. Only the amount of power necessary to keep accessories running and keep the battery charged is produced (if functioning properly).

 

As you point out in your first statement, the underlying phenomenon stems from Newton's Third Law of Motion, namely, For every action, there is an equal and opposite reaction. Light emitted in the beam causes a force in the opposite direction of the beam -- thus retarding the car's velocity and requiring more fuel ... less mileage ... et al.

However, your assertion that heat from the emitted light results in greater air friction is a discombobulation[1] and a distortion of Boyle's Law[2]. Let's set the record straight on that point.

One wonders whether HID or xenon beams would differ in their ability to enhance or detract from achieveable mileage???

[1] Discombobulation. A theoretical boundary condition allegedly deduced by Robert (Bob) Boyle who studied the relationship between pressure and volume of an ideal gas concluding p*V = constant. Some, including Boyle himself, wrongly argue that as the volume V approaches zero, then the pressure p must rise without limit ... a claim soundly refuted by most scholars.
[2] Boyle's Law

 

You might want to check your Blinker Fluid. It can affect your MPG as well...so Ive been told.

 

http://www.howstuffworks.com/question424.htm
http://www.motorists.org/drl/
Sun's up - light's off - thank you

 

Also, it needs to be noted that when the headlights are lit the electricity flow to the lamps create electro-magnetism on the wires. If the magnetic force field is in such way that the earth's magnetic force exerts any forward vector in relation to the car's velocity then the car's mpg will improve but if the force is exerted in the opposite direction to the car's velocity then the mpg will drop.
This might also contribute to the occasional rattle some drivers experience if the wires are not securely fastened to the chassis of the car.
The navigation system can be coordinated so that the on board computer can determine when to turn the lights on to take advantage of the electromagnetic effect as well.

 

From that link lets caluclate how much gas DRL use, assume (from the article above):
100 watts for bulbs
25% efficiency of internal combustion
70% efficency for alternator to 12V DC
1g gas contains 60,000 Whr of energy

60000Whr x 25% x 70% = 10500 Watt hour to the lights per gallon of gas taking into account internal combustion and alternator.

100w bulb /10500w/hr = .0095 g/hour to light the DRL

Now another assumption, say you get 34mpg running at 80mph average
this is 2.353g/hour

.0095/2.353 is about 0.4% hit to mileage or about 0.14mpg

Hope I got the math right

 

So after all that, Staz87 was right!

 

Well and truly noted ... moreover, the effect should be greater near the equator where the magnetic flux lines are at a more oblique angle and, hence a more pronouncded horizontal component than say near Hudson Bay where the flux lines are nearly vertical to the earth's surface.

Perhaps our colleagues in Texas could contrast their mileage v. our friends in eastern Canada to verify your theory. Surely these discussions will ferret out the relevant factors affecting mileage with or without headlights lit -- hopefully independent of whether S or non-S machines.

 

When I was racing Bugeye Sprites we were removing the generators entirely and replacing batteries with freshly charged ones between track sessions. A generator could sap a significant amount of power out of a 948cc normally aspirated engine. (Even though it was punched out to 998cc and cranking 100hp)

 

A lot of good and funny posts up there. But whats pretty sad is the education in other countries related to this such as India. Not sure how well people would believe me on this so i found something else talking about it: http://www.eol.ucar.edu/dir_off/proj...DOEX_log1.html

"Arrival in India is always late at night; Airport is about 20 miles out of town, cabdrivers do not use their headlights once they leave the airport "to save gas"."

Not sure about the current state but this was a big issue as cars were starting to get more common. People were warned at my work about it.

The .1 sounds about right. Certainly nothing to justify the safety issue of driving around without headlights on in the dark. Depending on your volume setting your radio could be using more energy than your headlights.

 

The absolute best response to a post I have ever seen!

 

Definitely go faster and see what happens. And I agree, the fresher the wash and wax the faster you will go and the better gas mileage you will get. It only makes sense. Why else would snowboarders wax their boards and surfers and stuff. I also read and article showing that headlights on does use more gas, but its very small like the .1 mentioned. But the drastic change was probably more air flow, traffic, driving style and other stuff like that

 

Apparently there is a formulation of headlight fluid that you add to the flux capacitor to create a vortex which reduces gravitational forces and wind resistance. I tried it and got about 200mpg. Great stuff.