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MAX Update No. 86: How Headlights Affect MPG

086LiquidLightHere’s another goofy graphic, fraught with symbolism; no, MAX’s headlights don’t have a fuel tank of their own, I’m just trying to make a point.

As one approaches three-digit fuel mileage—100 mpg and above—little things start making a big difference. Things you’d never notice in a 20 mpg car are spectacular in MAX.

For example, I have a switch on the dashboard that I could label, “More than 100 mpg” in the middle position, and “Less than 100 mpg” in the top and bottom positions. Or I could just label it “Headlights.”

Up is high beam, down is low beam, center is no beam.

The better your mileage, the greater (worse, actually) the effects of your electrical auxiliaries. Your headlights, your heater fan, even your radio, they all consume fuel (via the added engine power to drive the alternator harder to produce the extra amps) at a fairly steady rate…when they're on, that is.

Many folks think that car alternators have a steady load and produce a steady amount of electrical power. We read it all the time in promotional literature for on-the-fly hydrogen generators, those that claim “…your alternator’s excess electricity…” as their power source. Well folks, there’s no such thing as “excess electricity” from an alternator, no leftover electricity that’s going to evaporate if you don’t use it for something. Alternators convert engine power to electricity, and the more electricity you demand from the alternator, the more power the alternator demands from the engine.

My headlights burn roughly an eighth of a horsepower's worth of electricity (90 watts), I'd guess it's a quarter horse when alternator efficiency and mechanical losses (belt slip and flex) are included. A quarter horse at the crankshaft costs me close to 2-1/2 fluid ounces of fuel per hour. At cruise MAX burns about 60 ounces an hour (and gets about 100 mpg) so turning on my headlights knocks me down about 4%, or 4 mpg.

Does that sound like a lot? Let’s look at it from the other direction—I can keep the lights on for 50 hours on a gallon of fuel. That doesn’t sound too bad, does it? If you had a portable generator that would power two headlights for two whole days on a gallon of fuel, you’d have a pretty good generator, don’t you think?

And I’ll bet you never thought you could improve your mileage by 4 mpg by turning your lights off, did you? Good, I’m glad you never thought that, because you can’t. I can’t either, except when I’m driving MAX.

When I'm driving my 20 mpg van at 60 mph, I'm burning 3 x 128 fluid ounces of fuel an hour, so its headlights are roughly 0.0065 (2/3 of 1%) of its fuel consumption, roughly 1/8 of an mpg. Barely detectable. But it’s still an extra gallon every 50 hours, no matter which car I’m driving.

I drive with my lights on in the daytime (see the first photo in Update No. 80: Cooling Tests and Travels), because it makes MAX more visible…except during fuel economy competition. I shut them off during the Vetter Challenge because I figured riding with a pack of motorcycles made me plenty visible enough, and I really wanted that extra 4 mpg, but in my normal travels, I think DRLs (daytime running lights) are worth the fuel.

This winter I’ll experiment with LED running lights, and see if I can get the best of both worlds—high visibility with low electrical power.

Photo by Jack McCornack 

Browse previous MAX Updates. 

Check out the 100-mpg Car page for all things MAX. 

abbey bend
1/13/2012 9:44:42 PM

One more thing, the 1156 bulbs will not meaningfully interfer with your headlights performance when using them at night, they will just shine through the glass, with possibly a small faint shadow from the filament posts, which you will have to look for to see.

abbey bend
1/13/2012 9:42:02 PM

Jack, By placing the 1156 in front of the headlight, the headlight reflector keeps it from operating as a true bare bulb. A 1156 operates at 27 watts, makes about 402 lumens which is relatively bright, they are only running lights after all, and the headlight reflector behind them, directs the light to a certain degree, making them appear brighter than they actually are. H6024 lamps operate at 55 watts each on low beam, 65 watts each on high beam, and without the tail lights burning 1157s at 8.3 watts each, your wattage savings is about 72.6 watts, or around 57 percent savings. Total wattage of your headlights on is about 126.6, wattage of only two 1156s on is about 54 watts.

1/9/2012 11:36:30 PM

I stand corrected. I discussed this with my buddy who's been teaching electrics/electronics for Case IH for many yrs & says the increased magnentic load in the alternator as an accessory is switched on can account for as much as 50rpm increased load on the engine. For a vehicle running @3000rpm, that's in the range of 1%. For a typical car getting 20mpg, it's insignificant. For your application, that's another mpg.

paul compton
1/9/2012 1:42:51 PM

Automotive alternators are very much built down to a price. Efiiciencies could be 50% (peak!) or lower. Bosche were advertising special high efficiency versions that could achieve 75% at one time. Here is a link to a white paper on alternator efficiency and fuel economy ( . The very crude cooling fan is one problem and a NACA duct feeding a cooling shroud and backed up with a temperature controlled fan (Dell Optiplex 270 tower CPU fan would work well) would help. The second problem is the magnetic design. The 'claw' rotor is just plain cheap. A permenent magnet alternator could be as good as 95% although it would need a completely different regulator system that would have it's own efficiency loss. The combined efficiency should still be considerably better (80% range). Extend this idea a bit further and you arrive at theHonda IMA (Integrated Motor Assist) or stop-start technology where the flywheel is replaced by a starter/alternator which can also be used to lessen the load on the engine under acceleration, keeping it out of a poor efficiency area. Quartz halogen bulbs (which have much better efficiency because they can run the filament at much higher temperatures whilst still achieving good life) can be obtained down to about 35 Watt for classic cars where the original charging system is not suficcient for higher loads. HID (high intensity discharge) lights are also around this level of power consumption, but many of the kits are not road legal in many places.

jack mccornack
1/8/2012 2:05:04 AM

Abbey, that's an interesting thought, it would sure be simple, but I don't know if bare 1156 bulbs (those are the single filament turn signal and tail light bulbs from before the car manufacturers went to LEDs in the back) would be visible in daylight--I'll check it out. And T Brandt, cars with carburetors do slow down at idle when you turn on the headlights, and though many fuel injected cars compensate by increasing fuel flow when the headlights are turned on, many do not, and will lose some rpm when the lights go on. Try it some time; after a drive, sit at idle for a moment for the revs to settle, and turn the lights on...and off...and see what happens to rpm. Or if you have a ScanGauge, you can see idle fuel consumption go up when you flip on the lights.

t brandt
1/6/2012 11:33:09 PM

Yea, the alternator armature is spun and putting the same drag on the engine HP whether the generated juice is going into the battery or not. If we follow the logic of the article, then your engine should slow down when you switch on the lights. The AC is a little different in that the compressor is disengaged, therefore not putting a drag on the engine when not in use. A solenoid is employed to increase rpms automatically at idle to compensate when the AC is switched on.

john & virginia ledoux
1/6/2012 11:09:31 PM

Correction... A/C on, 30 MPG goes down to 27, windows down MPG stays the same. Sorry

john & virginia ledoux
1/6/2012 10:27:23 PM

This is the dumbest study I ever heard of. Being a mechanic/electrician there is no difference to affect MPG. The study of using your A/C or have your windows down to keep cool, is full of untruths too. Studies said to use your A/C to save gas.Use a vacuum gauge or on your dash the MPG gauge and test it. I did, windows up (example) 30MPG, A/C 27 MPG.

abbey bend
1/6/2012 3:35:09 PM

It would be simple to half or even more, you 4 MPG by just using a couple of 1156 bulbs instead of your entire lighting system as running lights. Cheap, easy to place by putting them inside of the headlight fairings, much as Volkswagen did on the old Beetles years ago.