Lights on = lower mpg?

Several clarifications ...
(1) 'Watts in an hour' is redundant. The unit 'watt' already has a time factor built in. Adding another time factor such as 'per day' or 'per year' creates ambiguity, leaving us engineers wondering which of multiple interpretations was intended.
(2) 55 watts is likely per headlight. It has two of them.
(3) Because we cannot turn on only the headlights (outside of DRL), we also need to include all the other lights that come on with it -- taillights, side markers, front running lights, dash panel lights, license plate light, and probably a few more that I forgot. I haven't inventoried the Prius, but on my older cars these auxiliary lights (4 taillamps instead of 2, none are LEDs) add up to quite a lot.

But I still keep them on all the time.

I've understood that in certain impoverished areas of the planet, this thing of saving fuel by not using lights is a common practice. But what they don't comprehend is that nearly all of the fuel savings comes from something else -- dead drivers and wrecked vehicles don't need any fuel.

 

Can DRL's be added to the Prius models that don't have them standard? I miss them after having them on my Outback! Like the Washington members have said, living in the Pacific NW means LOTS of gray days--and my silver car blends right in to the rain, cement pavement, fog and spray. I want to be able to see others also.

 

The mpg hit for low beams is 1% and high beams 2%. That's based on an analysis of headlight current draw vs. propulsion requirements which is about 200 watt-hours/mile. Wet road loss is much worse and has been attributed to the energy required to throw spray in the air. All are unavoidable.

 

Anything, consuming energy, affects mpg.
The questions is how much.
The lights energy consumption is watts-hour, constant energy usage time basis.
We use fuel economy unit using miles per gallon unit.
Let's change it to gallons-hour unit.
60 mpg @ 60 mph -> 1.0 gallons-hour consumption.
60 mpg @ 30 mph -> 0.5 gallons-hour consumption.

Therefore, the lights constant energy usage affects twice when vehicle speed becomes half.
My rough math about the effect of lights are 0.5% at 120 mph, 1% at 60 mph, 2% at 30 mph and 4% at 15 mph.

Ken@Japan

 

Not to mention the photons pushing against the car as they shoot out!

 

Whoa... I didn't know you were from HERE!! Well.. next door. We are in Hoquiam.

I run with headlights most of the time, unless it's those 3 weeks a year when there are blue skies and sunshine

We also picked a white Prius for more visibility, because gray or silver, even the red, is just invisible in the misty, rainy, cloudy, and foggy days that are so common here.

 

I spent from 1961 to 1981 in Montesano, but the article on Aberdeen had a better graph of the weather, and lets face it, all our towns are under the same cloud. In 1981 I moved to Bellingham for 7 years. In 1988 I moved to Elko Nevada and only visited on Thanksgiving week. In 14 years, they sank two floating bridges with rain, Thanksgiving week.

Grays Harbor Weather Perpetual report: It will start raining the Thursday before the first high school football game. It will rain until December 27 (missing a white christmas) and snow one day.* Then it will rain until July 5, ruining fireworks. Then it is sunny until the Thursday before the first high school foot ball game. (no one ever gets to read the numbers on their uniforms)

And folks wonder why I buy cars that you can always have the headlights on, never need to turn them off.

* Once a decade, the wind will be blowing from the east on Dec 27, and the snow stays a week.

 

I can't detect a difference as it is so small. I have headlights on whenever I drive. I suspect the OP is a more agressive driver than his girlfriend.

Have the girlfriend drive with headlights always on and check her results.

 

For the past two months, it has been dark in the mornings when I leave for work and dark in late afternoon when returning. The last week or two, as the days have finally begun to get longer, it has still been light when I have returned home.

So, for a couple of months I had headlights on both ways. Now I only have to run them in the mornings. I cannot tell any appreciable difference in mpg. Of course, there are so many other variables that come in to play that is really hard to say.

 

The first Tacoma Narrows Bridge, Galloping Gertie, collapsed in the same month as Thanksgiving too.

Any bets on which bridge failure will be next? Two leading candidates are the 520 floating bridge breaking up in a windstorm, or an earthquake taking down the Elliot Bay seawall and collapsing the Alaskan Way Viaduct, similar to the Embarcadero highway in the Loma Prieta quake.

 

The IV, with the "heavy sunroof option", weights no more than the V with it's heavy (bigger) 17" tires and rims. LOL

But running the parking lights has other advantages (besides fuel savings) than running the headlamps :

(1) Many of today's (whiter/brighter) hologen bulbs burn out as quickly as 6 months. Read other threads/web sites and you will see this from many posters. A common complaint among halogens. Having to replace two halogens every 6-12 months, is an added expense and time to that I would prefer to prolong as long as I can. The LED parking lights will last almost the lifetime of the car.

(2) If I run the parking lights, the rear tail lights are also on...and both automtically go off when I turn the car off. In the headlight position, they stay on for 30-60 seconds after the car is turned off. Not a big deal, but I'd just as soon they go off when the car is turned off.

Maybe some drivers just can't see as well as others, or it could also be our definition of "mist". I have no problems seeing cars with even no lights on on misty days. Further, i have a Pearl While Prius...studies have shown that light colors, like white, yellow and silver tend to stand out - much better - against a dark/grey background. That's why school buses are yellow.

Ummm, maybe "auxiliary" lights was too long/difficult for people to say?

 

Have to agree with you. During the summer you would take a max hit with AC and lights on and yet that is when you get your best MPG

 

This is true, ergo, the "daytime running lights".

The same phenomena was studied when cars went to having the third brake light (positioned higher up in the rear window). But studies have also shown that as more and more car have their lights on, or have the 3rd brake light, that our brains have accomodated and gotten use to this, and they (more or less), tune it out. Don't know the psychological term for this, but it's very common for humans to notice something new in our environemnt, and when it becomes common place, "tune it out". I'll even go so far as to say that I notice a oncoming car WITHOUT headlights on, far more than I do the rest of the traffic with headlights on.

If you really want to be noticed - drive with your high beams on, or better yet, flashers. I guarentee you that you will be noticed.

 

This is true. It's also true that the early positive tests of daytime running lamps were done in the winter in Scandinavian countries. Scandinavia is very far north, and their winter daylight is more like our twilight. The contrast between headlamps and the darker sky works much better than headlamps and a bright sky.

Subsequent test in brighter locations have had mixed results. Further muddying the results is the fact that some of the follow-up test were not unbiased.

It seems logical that headlamps help make a car more visible, but it also makes sense that the benefits diminish when *all* of the cars are running around with lit headlamps.

As an example of how counter-intuitive this can be, in WWII anti-submarine aircraft used lights during the day to make themselves nearly invisible. A string of white lights along the front of the aircraft lightened the aircraft to make it blend in perfectly with the bright sky. Daytime running lights only work if they contrast with the background, not blend into it.

Tom

 

Our brain is pretty good in pulling out things that are different in a scene. Computers are even better. But once the scene become (esentially) static and monotone (e.g., with all the car headlights on), the brain tends to filter out, and push back in our recognition process.

As I said, if one really wants to be noticed, put on your flashers. You will stand out from every other car. Or even run with your high-beams on - in the daytime. To proponents of always running with your healights on...if the issue is safety...then why not turn your high beams on during daylight hours?

For years, bicyclists have used flashing lights/reflectors to warn motorists of their presence. Far more effective than having some bright, yet static, red/yellow light on their bike. It's the change in intensity that people notice. Also why police, ambulence and fire cars/trucks modulate all the lights on their vehicles as they speed down the road.

 

Because it can cause blinding glare even in daylight, and legally must be turned down for oncoming traffic just the same as at night.

 

The example I saw, to hide tanks instead of aircraft, had dimmer non-directional lights more closely spaced (in angular measures) than automobile headlights.

The camouflage lamps ought be close enough to nearly blend together from the distance of the observer. At the distances we need oncoming drivers to see us, headlights are very easily resolved into separate lamps by anyone with legal driving vision.

 

Sure...and I can ALSO get "blinding glare" from oncoming cars with their low beams on. I've had it happen to me many times - under the appropriate conditions. For example, road mist in the air will act to refract the light from oncoming cars with even their low beams on. In this regard, running with my parking lights (when it's still light outside) will not produce any blinding glare to oncoming drivers. But even if you run the low beams, you do run the risk of blinding the oncoming driver in misty conditions. I'm not advocation driving with the high beams on. In fact, and most of the daylight hours, I run with my parking lights.

One fact you need to consider - automotive lighting (brightnesss, light specrum and beam patterns) have changed dramatically over the past decade. Today's headlights are nothing like your dad's headlights...or when many of these codes were written.

As to the law, well, that depends on your state. That said, there are many, many laws on the books that are never enforced (nor could they be enforced) by the police, with that being one of them. In fact, now with more and more cars coming with super-bright HID headlights, it's often difficult to differeentiate the guy with brights on, from the guy with mis-aimed low beams, or the guy with HID lamps. Only the color of the light will identify the HID driver.

 

But still, in this thread, just turn on your lights at dust and in the rain instead of "saving" the miniscule amount of fuel by going with just the parking lights.

As for the third brake light, we wouldn't need them if the US just adopted the yellow rear turn signal the rest of the world as adopted. Also the practice of using one light for tail lights, brake lights and signal lights should be outright outlawed today. Ridiculous, how much could the wiring cost to separate the functions?

 

If it was only about mpgs, I would elect to turn on my headlights. But it's more than that.

You missed my point. In fact, you completely ignored the posts (not just mine) regarding how changes like this, when first introduced into the market, does help to improve safety...but as time goes on, and people get use to it, the impact of the third light, or turning on your headlight in the daytime, has greatly diminished impact. If I can find the studies, I will show you proof...

I have seen no studies that compare headlights with parking lights. But as I already mention, if visibility is a concern of your's, then I hope you bought a white/light-colored car, because lighter stands out much better against the grey background of the road and a cloudy/rainy day? Fact.