Does Using Fog Lights/Other Electrical Accesories On Your Car Reduce Mileage Significantly?

Tonight I was at my parents house for my son’s birthday. I own a 2008 Mustang GT. I put all our stuff into the car and started it up to warm it up a little before we left. I turned on the headlights and the large-ish foglights mounted in the grille.

My Dad saw my car in his driveway and said “why do you have those extra lights on?”

I replied “I get better visibility/more light and I like the way it looks”

He said “You’re really hurting your gas mileage doing that”

I was like “Really? Those bulbs aren’t very large, just the housing is. What, does it make the alternator run that much harder?”

Him: “Yes”

So…from a little googling, my alternator is rated at 135 amps. Pretty stout, I think for a stock alternator.

So my question is: does using the foglights unnecessarily really impact mileage all that much? I think my Dad is crazy. I understand that any additional load on the alternator will impact mileage slightly…but foglights? Really?

This has to be something that’s miniscule in scope and the car gets shitty mileage anyway…so how much am I really affecting it?

Closest (and largely undecided) Cecil reference:

I agree that it’s probably negligible. But for the hell of it, here’s some back of the envelope figures:

From Wikipedia, headlights seem to consume around 50 watts. Times two, and that’s 100 watts which makes the math easy. So to run your headlights for an hour, you need 0.1 kW * h.

Gasoline has an energy density of 37 kWh/gallon. By WAG, let’s say that the energy is converted into electrical power at around 20% efficiency (25% for engine, 80% for alternator sounds reasonable). So your hour of headlight use burns 0.5 kWh worth of gasoline, or around .5/37 ~ 0.02 gallons (rounding up for pessimism’s sake).

Now, your car gets somewhere around 15 mpg in the city. I’ll assume city driving averages 20 mph (including all the time spent stopped) so you consume ~1.33 gallons per hour. Adding headlights, you consume 1.35 gallons per hour. Converting that back to mpg, 20 mph / (1.35 gallons/hour) = 14.8 mpg.

That difference is probably measurable under controlled conditions, but is otherwise negligible. Especially compared to the less-than-fuel-efficient driving habits that are encouraged by such a car…

i have heard that only the window defroster and the AC make any significant difference, and even then it’s pretty small.

the H11/9145 halogen bulbs in the Mustang’s fog lamps are about 55 watts apiece, so with both lit, you’re consuming just about 1/7th of a horsepower (not considering inefficiencies.) So yes, you are using more gas to power them, but whether you’ll be able to actually figure out how much is up for debate.

the other thing is that the Mustang actually has proper fog lamps with the desired beam pattern (wide, sharp, bar-shaped beam,) and aren’t likely to improve visibility much if at all in clear conditions.

Put it this way: Even if you could accurately measure, down to the fraction of a MPG, your mileage and you drove the same route both with & without your fog lights on, the difference they might make would be less than the difference caused simply by how you drove (i.e. how often & hard you accelerated, coasted, traffic etc.)

IOW it’s just a dad thing, it makes no practical difference.

Do you have heated seats? Because that’s usually the main reason to have a 100+ amp alternator.

No. Its a GT Premium, but no heated seats. I assumed from googling that the 135 amp alternator was just standard. But the capacity of the amperage production of the alternator and the actual usage in terms of gasoline in the operation of said alternator aren’t the same thing, are they?

No, that’s just its maximum output. A high output alternator in and of itself isn’t a greater strain on the engine until its actually drawing high amps. And then, as stated, it still isn’t as big a strain as A/C or even power steering (when you’re turning the wheel at least).

Also, unless it’s really really cold out modern cars don’t need to be warmed up anymore. The fuel system is computer controlled fuel injection and the engine blocks are aluminum alloy.

The amp rating of the alternator is a maximum rating. The various electrical devices in the vehicle will only draw as many amps as they need. Unless you have the lights on and the stereo cranked and everything turned on in the car, the actual amps supplied will be significantly less.

High capacity alternators are also common on cars with high power stereo systems.

True, but you should take it easy for the first few miles.

Also worth noting that a gasoline engine, especially a large displacement one which I would expect a performance car to have, is normally operating in a very inefficient high manifold vacuum state except when climbing very steep hills, or under hard acceleration. In this condition, incremental load increases have very little effect on fuel consumption. This is why running the AC (a much larger load than the lights) usually has little effect on gas mileage of a muscle car. The incremental load reduces the manifold vacuum, which lowers pumping losses, which improves efficiency. Without detailed performance curves, and knowing the load on the engine it is impossible to be exact, but a fair WAG would be that of the 100W consumed by the headlights, probably around half (and I’d wager more than that) will come from lowering the pumping losses…fuel that would have been burned anyway.

Question answered, so I’ll add a plea:

If you’re always driving around with fog lamps on, please make sure they are adjusted properly so that they aren’t blinding everybody you pass or drive behind.

Also consider that lights that light up the foreground may make you feel like you are getting better visibility, but without throwing more light into the distance down the road (which fog lights do not really do), you are actually getting less far distance visibility, the visibility you need for anything over parking lot speeds.

This is similar to the effect of having your dash lights too bright, too much bright light up close hampers your ability to see the far range of your headlights.

As for gas usage, pretty negligible, but it could be straining your charging system, which may cause premature failure or even perhaps a dead battery given the right set if circumstances.

Underlining mine. I’m not sure why you said this. The overwhelming majority of engine blocks are cast iron. Most cylinder heads are aluminum (not alloy, as far as I know), but neither aluminum heads nor aluminum blocks have a bearing on not having to warm up the engine. It’s the precise fuel metering that modern fuel injection affords that makes the difference.

Using the AC may actually be more gas efficient if the alternative is opening the windows and driving down the highway at 65mph (increased drag).

The car does have the “Shaker 500” audio system. I’m assuming there’s an amplifier somewhere in the car to power the 8" woofers in the doors. Maybe that’s the reason for the higher-output alternator.

They’re aimed however the factory aimed them. They do sit higher up than most fog lights though…they aren’t located below the headlights like on many cars, rather, they are in the front grille in between the headlights and on a level with them, so it’s like having four lights in a row almost.

I hear what you are saying…but I get a lot more light with the fog lights on in all conditions for normal darkness driving. I’ve experimented with turning them off and driving with just the headlights on very dark unlit roads and the difference is quite significant.

Are they white or yellow?

It’s you. You are That Guy.

the subwoofer amplifiers are behind the kickpanels. But that “500” is peak power (i.e. marketing power.) In reality, the system is 4x20 watts (from the radio) and the door woofer amplifiers deliver 2x55 watts each, so about 300 watts RMS total.

They are white…the same color as the headlights.

Sweet! Seriously though, to me, more light is better. And I don’t think that even with all four lights on its as bright as some of the HID lamps that are on many cars out there today. Those things are really, really bright.

Oh yeah, I figured as much. I once owned a “stereo” car back in college that I’d sank an enormous amount of money into…I learned back then the difference between RMS and peak power…and also back then I think amp manufacturers were able to fudge their power claims versus today. My system then had a sub amp wired to mono at 300wx1 pushing two Kicker 12’s and a 4-channel amp that was 30x4 that powered everything else and it was LOUD. 30 watts may not sound like much but when its true wattage and a clean, 5v signal…its plenty loud.

Why don’t you read any of the links that have been posted? Your foglights don’t help you on a clear day because they are not pointed at the right place. Brightness has nothing to do with anything. I’m sure your car’s instruction manual will tell you this also. Is it so hard to follow them instead of randomly mashing buttons on your dash?