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?

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.

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.

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.

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.

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.

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.

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.

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. ;)

Just continue to adjust the general concept for the specific conditions you have.

All else being the same:
~40 MPH + ~40 MPG + ~500 Watt Lights = ~5.7%
~40 MPH + ~40 MPG + ~250 Watt Lights = ~2.8%
~40 MPH + ~40 MPG + ~125 Watt Lights = ~1.4%

~40 MPH + ~20 MPG + ~500 Watt Lights = ~2.8%
~40 MPH + ~20 MPG + ~250 Watt Lights = ~1.4%
~40 MPH + ~20 MPG + ~125 Watt Lights = ~0.75%

~40 MPH + ~80 MPG + ~500 Watt Lights = ~11.4%
~40 MPH + ~80 MPG + ~250 Watt Lights = ~5.7%
~40 MPH + ~80 MPG + ~125 Watt Lights = ~2.8%

~80 MPH + ~40 MPG + ~500 Watt Lights = ~2.8%
~80 MPH + ~40 MPG + ~250 Watt Lights = ~1.4%
~80 MPH + ~40 MPG + ~125 Watt Lights = ~0.75%

~20 MPH + ~40 MPG + ~500 Watt Lights = ~11.4%
~20 MPH + ~40 MPG + ~250 Watt Lights = ~5.7%
~20 MPH + ~40 MPG = ~125 Watt Lights = ~2.8%

etc ... etc...

Faster average speed decreases the impact.
Lower average MPG decreases the impact.
Lower Watts of electrical usage decreases the impact.
Higher ICE average efficiency decreases the impact.
Higher alternator efficiency decreases the impact.
If cycled through the battery the higher the battery cycle efficiency decreases the impact.

I don't have DRL's but I do run with lights on all the time - its amazing how it reduces the number of people 'not seeing you' and pulling out.

The other side of that coin is that pedestrians don't have DRLs and get less attention time.

I grew up in a world where that was considered a legitimate reason for using the HORN. :p

Yes, I have installed air horns, and they are effective at waking up brain-dead drivers when needed.

Street lighting is much better here than in most countries I have been to including the US, but still those of us with kids do fit them with the equivalents sometimes even in daylight :D

http://www.workwearexpress.com/image...1-85055400.jpg

I find my approach is less annoying to others not involved in the situation. I've used my (car) horn probably less than twice a year. :cool:

When we've covered everything in fluo, nothing will stand out anymore.


I've had my DRL (which uses the headlights) deactivated by the dealer.
I switch on the lights if and when needed, as I had been doing the 15 years or so before.

DRL have become mandatory fitment on new EU vehicles this year.
These are now usually lower-power LEDs.

How did we get by without DRL's for nearly a century?

DRL's add to the cost of the vehicle. Their imposition certainly benefited bulb manufacturers. I say "imposition" because without providing a switch to turn them off that's what it is - an imposition.

I have no problems with DRLs, they seem much better than the alternative which is muppets in heavy rain or even at night operating in "stealth" mode.

Silver Audi, Fog, no lights, 90 mph on the motorway = not clever.

The "no lights" in precipitation and "90 mph" parts are already universally illegal. When everything is made "idiot proof" we will be living is a world where no thinking is required. Unfortunately that trend tends to build upon itself and there will little incentive to be anything more than an idiot. When everything is on auto pilot the need for critical thinking is eliminated.

I tested this on my car and saw about 2.5% difference. Enough to make me turn them off during the day. The science equations back up my findings.

http://ecomodder.com/forum/showthrea...hts-15179.html

What about efficiency loss?

Dont' run with my lights on when I'm in a car or truck except when conditions require. But when on the motorcycle, they're always on. Always thought of DRL as a nuisance.
VT247

When Poland switched to mandatory lights 24h/year round I saw newspapers usually citing 2% as the increase in fuel consumption. I decided to install LED DRLs (2 x 5.5W = 11W) since in order to drive with my lights I'd not only have the headlights (2 x 55W), but also position lights, registration plate lights, dashboard lights, etc., plus all the relays that power those lights, which I figured totalled to ~200W. When the engine is warmed up I see instant fuel consumption at idle go from 0.50-0.53 l/h to 0.56-0.64 l/h when I turn the lights on.

As for whether DRLs increase safety I can say that yes, they do. First of all, on a sunny day (when many people think that DRLs don't help) some cars tend to blend into the pavement and/or horizon when far away. I can spot them much sooner if they have lights.

Second, among the many dumb games teenagers like to play is driving at full speed on the highway. Against traffic. At night. Without lights. If their car has DRLs which can't be turned off, then that increases the chances of any innocent drivers that might be unlucky enough to be on that stretch of highway.

I'm with you on the first, but making it more convenient to pass under some circumstances doesn't really make me want to require equipment across the board. Your second reason is a little out there- mandating equipment to make one part of a criminal activity a little less convenient just doesn't make sense to me. If the kids like doing it without lights then they're going to do it without lights no matter what you make everyone else buy.

It makes a big difference if you EOC. It can make the difference between starting or not at the next red light. If you have to engine-ON-coast instead to keep the alternator running it costs you a lot more fuel. Something in the 20% range I'd guess.

OK, I was half joking with the second reason;)

If it was this much I would expect to see it on the GPH figure at idle and turning my lights on or off makes no difference that has stood out so far - I think I will experiment tomorrow morning... :turtle:

What's your idle gph? Mine's about 0.3. Compare that to the gph of EOC = 0. Now apply that difference to half of the drive that I EOC. It makes a big difference.

Estimated 0.3 * (0.5 hour drive * 1/2 (eoc percent)) = 0.075 gallons
Regular commute = 0.14 gallons (10.5 mi / 75 mpg)
0.14 + 0.075 = 0.215
10.5 / 0.215 = 48.8 mpg if I engine-on-coast instead. :eek: That's a 35% drop!

I forgot to check this morning as we were in a hurry. Just went out and idled for a bit and its on 0.25 which is higher than normal - probably because it has sat at work all morning and isn't warmed up. I shall check again later.

Normally it is under 0.2.

As I mentioned before, and it is the sort of thing I will probably have to mention again, since it is the sort of thing that is surrounded by a massive SEP field (Someone Elses Problem), but extraneous lights on cages in the daytime take attention away from pedestrian traffic and more pedestrians get hit as a result, is my current hypothesis to explain why DRLs result in more pedestrian deaths.

Here was what I came up with originally:

Idling GPH and LOD figures. Helga is a TDI, headlights are Xenons, SG2 has been calibrated for gallons ~ 4.54 litres (imperial). Let her idle for 30 secs when already warm to get the figures settled and then idled for another 30 seconds both for lights on and off to let the measures settle.

I didn't do the 30 seconds idle before just an instant on/off which didn't really show anything.

Lights off - LOD varies slightly between 20/21 - mostly 20.
GPH is 0.14-0.15, again mostly 0.14.

Lights on LOD stays at 20/21 but mostly 21.
GPH is still 0.14-0.15 againt mostly 0.15

Thats a 5-8% difference which is worth investigating further. My current tank only has 50 miles on it so I'll avoid the lights as see if it really makes a substantial difference.

Remember that the headlights' load is proportionally larger at idle than when driving, so unless you plan to A-B-A test it, the results will probably be within tank-to-tank noise.

That article also mentions an "insignificant" increase of %4 more motorcycle accidents when cars use DRLs

Day Rime Running-lamps are not the same as automatic headlights, you can have one and not the other. I have found it much better to have automatic headlights than DRLs. Several cars that have DRLs reduce the power sent to the lights mostly for bulb life. My GrandPrix and a few other GM cars all you have to do is unplug a tiny box under the drivers side dashboard to disable DRL the automatic lights still function properly and if needed the lights can still be turned on manually.

Also I believe the reason that cars in the USA have DRLs is it is required in Canada so all models also sold there have it installed by default (atleast for GM).

I may try and figure out an A-B-A test as the difference at idle was higher than I thought. Helga has a lightened flywheel compared to standard but its still heavy compared to a petrol car.

Based on this I have assumed the effort of turning it would be greater than any load - certainly using the A/C (which I have done 3x in the last 5 years) never had any effect I could detect in terms of performance. I never used it long enough to determine an effect on FE.

I deliberately turn mine on, I don't have DRLs or auto lights, just a switch by the wheel which I use to turn on the dipped lights. I started doing this when I had a FIAT which allowed it and the number of people who would pull out, do a double-take and choose not to was quite amazing, so I have run with headlights ever since. Around the same time I worked with a biker (as in MC) who said he always ran with lights otherwise he seemed to be invisible.

Every MC I see these days uses lights too, see my comment above.

I agree though, if everything stands out then nothing does.

It is the law here to leave your motorcycle light on, and it apparently made bikes 4% safer before cars started turning their lights on during broad daylight too.


Yes, and nobody is trying to make pedestrians stand out or have pedestrians/bicyclers compete with 100+amp alternators on cars, so bikes and pedestrians lose when cars all decide to use DRLs, not your problem I guess.

not gonna fly, driving is a privilege, walking is a right.

Indeed. The State will determine what your "rights" are.

Once anything requires a license from the State, it then becomes a "privilege".

Get that mandatory, tax-generating, air flow meter back on your nose, buster, before the "Air Police" (a task force of the EPA?) get here! :eek:

Lol, with appropriate penalties for mouth breathers. I hear they will jump start the program and initial equipment investment with seed money from all the traffic enforcement cameras.

Might help prevent Pavement Rage. :D


This is a little over simplifying it I feel. :cool:

In "Britain" there are two laws here - implied consent and the right to roam. Implied consent means that people carrying out their peaceful business are allowed to enter my property - so the postman can deliver stuff through the letterbox, UPS can knock on the door etc. As a property owner I can withdraw consent to any specific person or legal entity at any time. If I secue my property than nobody is allowed to break in.

The right to roam governs access by ramblers to open land including farmland. This is given on the basis that those doing the roaming do not cause any damage to crops, disturb animals etc.