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Old 05-19-2011, 03:46 PM   #1 (permalink)
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keeping lights off, how much fuel does it save really

I always see people here advocating to keep your lights off unless it's needed,and even offering advice on how to defeat DRL circuits, because it apparently saves some fuel. But how much fuel is really saved?

I ask this question because it's been rainy in Ann Arbor lately, and I see a lot of people drive with their lights off in the rain even at dawn/dusk. I hate this and I think it's a huge safety issue to drive with lights off at all - I /always/ drive with my lights on even in broad daylight so other drivers can see me better.

I'd rather take the (I think minuscule) mileage hit by leaving my lights on, than be unsafe. Isn't that why there are DRL laws in some jurisdictions?

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Old 05-19-2011, 05:17 PM   #2 (permalink)
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I've seen some 1% estimates. You should do what you feel is safe for you. I think the added safety from lights on is less than the fuel benefit from lights off.
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Old 05-19-2011, 05:37 PM   #3 (permalink)
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On my Honda Civic, it drops my mileage by 2-3 mpg on trips averaging in the low 70's mpg. On my F-150 at 21mpg, the mileage drop is hardly noticable.
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Old 05-19-2011, 06:07 PM   #4 (permalink)
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Quote:
Originally Posted by SentraSE-R View Post
I've seen some 1% estimates. You should do what you feel is safe for you. I think the added safety from lights on is less than the fuel benefit from lights off.
I've seen studies that suggest DRLs are more dangerous for pedestrians. Though I do believe having your lights on in the rain is a good idea.
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Old 05-19-2011, 06:45 PM   #5 (permalink)
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DRL FTL.

I've never liked them. Most times they're too bright, especially for some reason on Saturns. The whole premise behind them seems wrong, too: there's absolutely no reason to "not notice" a car that is being operated safely. I disabled mine not for any safety reason but because I don't like having lights on my car that I don't control.
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Old 05-19-2011, 08:03 PM   #6 (permalink)
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Quote:
Originally Posted by Incienso View Post
I see a lot of people drive with their lights off in the rain even at dawn/dusk.
Those people are called morons. They typically are driving a car that blends into the pavement as well.

DRLs were created because of morons who drive 24/7 without lights (yes they do exist) Cars drive all hours around town with no lights because they can see via the road lights.

If I have my lights shuttered at night it was usually because my alternator failed and my battery is going dead. (or because I am coasting a long distance but in that case the lights are on the instant other traffic is present)

Anyway I feel the same way you do, if its dark use lights. If its bright and sunny leave them off.
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Old 05-19-2011, 09:08 PM   #7 (permalink)
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You could adjust the ~36kwh per gallon energy content of the gasoline to account for some general efficiency losses in the system ... ~30% efficient ICE brings it down to ~11 kwh of mechanical energy ... maybe ~80% efficient Alternator brings it down to ~9 kwh of electrical energy... even if you don't go through a battery cycle you are down to ~9kwh of electrical energy consumes an extra 1 gallon... of course the more you know about your particular system and operating conditions you could teak for a more accurate number for your specific car and conditions.

How much energy are the headlights consuming? Varies from car to car but maybe around ~200 Watts .... which would take ~45 hours of operation to equal ~9kwh or about ~1 gallon worth of gasoline ... even ~500 Watts of lights would take ~18 hours for ~9 kwh or about ~1 gallon of gasoline.

Once you know roughly how much energy your headlights consume and roughly how many kwh of electricity you can expect per gallon ... you just need to know what your average speed was over that period of time.

If you averaged say ~40 MPH over the 18 hours of 500 Watts of lights ... that is ~720 miles of travel to loose ~1 gallon.

If over that ~720 Miles you averaged say ~40 MPG ... you used ~18 gallons ... 1 of those 18 was from running your lights ... if you had traveled the same distance ~720 miles under the same conditions without the ~1 gallon worth of lights ... you would have only used ~17 gallons to go ~720 miles or ~42.3 MPG... or about ~5.7% increase.

Of course YMMV , depending on the specifics of your ICE / alternator efficiency ... lights consumption rate ... the distance you traveled over ~1 hours time ... and the average MPG you had over that ~1 hour and distance.

For example ~250 Watts of lights ... at the same conversion efficiency ... over the same distance ... for the same period of time ... at the same MPG ...etc ... would only have been a ~2.8% increase.
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Old 05-19-2011, 09:48 PM   #8 (permalink)
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IamIan, standard halogen headlight bulbs are only 55W each
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Old 05-19-2011, 10:07 PM   #9 (permalink)
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Quote:
Originally Posted by winkosmosis View Post
IamIan, standard halogen headlight bulbs are only 55W each
Good point, and good analysis above too.
DRLs are typically even less than the low beams, and don't include any other lights.
200-250W is, however, probably an accurate estimate for regular headlights once you factor in parking lamps, taillights, and the myriad of tiny 921 and 912 lights for side markers, interior illumination of dash equipment, ash tray, license light, etc.
New cars are probably less due to LED usage.
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Old 05-19-2011, 10:33 PM   #10 (permalink)
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It's pretty easy to put your DRL's on a switch so you have control of it. I've done so, not so much to save fuel but to save the bulb life of them.

So I leave the switch on when it's raining (because they are good for that and it's convenient to have them come on automatically) and I shut them off when it's sunny. DRL's are not essential when the sun is shining.

My car, my choice.

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