Comments on: Fuel Economy Tip – Don’t Use Your Headlights During the Day

By: John DeRegnaucourt

John DeRegnaucourt — Mon, 09 May 2011 15:28:38 +0000

Daytime running lights make it harder to see those who do not have daytime running lights on. So the possibility that using daytime running lights increases accidents needs to be quantified. Furthermore, if every vehicle on the road were to have daytime running lights on, would we see the accident rate be the same as if they all were off?

With gas a $4.00 per gallon in the US, the savings is likely more than $10 per year. And for ‘Rafal’ he does not understand basic mechanics nor physics. Energy is not ‘free’. Turn on all headlights, crank a radio with a lot of base, and plug in a device into the cigarette lighter. You will see that the engine runs hard to power these devices, because of the extra load the alternator.

Look at the battery / alternator required for those who use massive steroes. If electricity were free (and was not converted from gas) then these specialized sound vehicles would not need to modify the size of their alternator.

If we quit looking at this from a single car POV, and begin looking at this from a global view, everyone running with their daytime lights on is wasting significant amounts of fossil fuels, polluting our atmosphere, and making our air harder to breathe.

By: Jim970

Jim970 — Sat, 02 Apr 2011 22:25:25 +0000

I find having lights on all the time to be an advantage. I know for a fact that when on a bicycle, with the small rear view mirror on my helmet, cars coming up behind me are MUCH easier to see if they have their lights on. I suspect the TV program you saw had an agenda they were pushing.

By: Mark

Mark — Sun, 13 Mar 2011 04:47:56 +0000

Hey Trevor,
I’m an electronics engineer and lights use energy. That energy has to come from somewhere, and the only energy input to the car is from fuel. If the battery supplied the energy it would go flat pretty quickly. Try leaving the lights on with the engine off and see what happens.

By: Jamie

Jamie — Wed, 29 Dec 2010 23:51:16 +0000

The alternator puts the same load on the engine whether you are using the power it is outputting or not, turning off you lights will not do anything to save your fuel mileage.

I really don’t understand where people get the notion that by using electric devices in their vehicles, they are using more gasoline. The gasoline will be used whether the devices are provided with power or not, the load from the alternator doesn’t change.

By: Jamie

Jamie — Wed, 29 Dec 2010 23:48:21 +0000

I agree, people don’t realize it’s not about being able to see, its about others being able to see you. When I drive my car, people have a terrible habit of not noticing me, either because the car is small or silver and blends in with the road.

Either way, “Lights on for safety!” is my motto.

Also, I really don’t understand how they figure that using less electricity in your car will use less gas, it’s not like the alternator is harder to turn when you are using more energy, its the same whether you are drawing all the amps it can output, or none at all. Seems like a marketing ploy to me.

By: Rafal

Rafal — Mon, 06 Dec 2010 22:49:52 +0000

i think that the battery does a little more than just smoothen out the current. It is a capacitor, yes, but a very big one that can deliver a lot of charge very quickly when needed, and while your statement about the alternator is true, in modern vehicles the over-sized alternator / battery combination really masks any increased load so it is almost imperceptible to the engine. So I really do not think your vehicle will burn any extra petrol just because you drive with your lights on all the time

By: Rafal

Rafal — Mon, 06 Dec 2010 22:42:41 +0000

Alejandro, the alternator wastes so much power that what would be an increased load on the engine simply becomes just a smaller power loss by the alternator. So when you turn your lights on the current tthat would otherwise be wasted goes to the battery and the lights and everything else connected. when you turn your cars lights on you are choosing to lose electric power through the light bulbs and not throu the alternator. So I’m afraid that driving with your lights on will cost you nothing extra in petrol but only in a few more burnt-up light bulbs.

By: B Mukly

B Mukly — Sat, 09 Oct 2010 06:29:10 +0000

Save $10 a year in gas, raise your risk of dying. I will pay my $10 per year and then some if necessary.

Lights on day and night.

By: Alejandro

Alejandro — Sat, 09 Oct 2010 03:52:36 +0000

Trévor you are completely wrong.
You say “The speed of the motor does not change when we turn our head lights on”
The speed of the motor might not change, but the load on it does.

This is a basic law of physics, “Energy can not be created nor destroyed, it can only be converted”

So if the only energy we put in cars is in the form of gasoline, then everything, ABSOLUTELY everything in the car is powered by this gasoline. It may account to trivial amounts of gasoline, perhaps a few gallons per year, but gasoline in fact powers everything in your car, even the tiny LED on your dashboard.

By: Trévor

Trévor — Sun, 27 Dec 2009 21:04:29 +0000

Just because someone prints something and ads their rational to something doesn’t make it true. The physics behind elecrical generation in a car are fundamental. Driving with your headlights on will not increase your fuel consumption one iota. Any mechanic or auto engineer knows that the generator or alternator turns in direct relationship to the speed of the cars motor. A fast moving motor will drive an alternator or generator at a higher speed. Thirty years ago an idling motor with a generator would produce just enough current to keep headlights lit. However, today when we design alternators and balance them with a battery we ensure that the electric load being drawn from the battery is less than that being generated by the alternator. In short when we drive our car it is always generating far more current than we need. The speed of the motor does not change when we turn our head lights on. The only time someone will experience a change in motor speed due to electrical current load is at idle, and not when driving the car. We design electrical generators to generate way in excess of the amount being drawn by a load such as headlights from a battery. If this wasn’t the case we would find we had a flat battery at the end of our journey and would be unable to start our car. Even then it wouldn’t increase the load on the motor cars engine.