MAX Update No. 86: How Headlights Affect MPG

Reader Contribution by Staff
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Here’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

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