Another quick question

I'm sure I heard someone on the radio the other day say that a car uses more fuel if driven with the headlights on.

Can this be true? Call me a techno-thickie, but I thought the lights were powered by the battery, which was re-charged constantly as the car moved. I drive west in the morning and east on the way home, so the cars coming towards me tend to be blinded by the sun around this time of year, so, I drive with the lights on full for safety's sake.
(Che Grimsdale Best before:, Mon 27 Oct 2008, 15:17, 28 replies, latest was 16 years ago)

Yes
If you leave your engine idling for a period of time, you'll hear the engine cooling fan cut in. This normally results in a small drop in engine RPM and the interior lights might dim slightly.

This is because the alternator (which provides electrical power to your car) is demanding a larger share of the power your engine produces which would otherwise be going to the wheels.

I wouldn't worry about the lights though, IIRC low beam uses something like 60-65 Watts, while the average car engine produces something like 80 KiloWatts. It'll only use a tiny amount more fuel.

Air conditioning and heated seats will steal much more energy than your headlights.
(Bicycle Repairman "you're also a bit of a wanker", Mon 27 Oct 2008, 15:31, Reply)

At risk of displaying my ignorance...
I thought the lights were powered by the battery, which was re-charged constantly as the car moved

Allow that this is true. Still, the radio'd be taking its power from the battery, and so the battery would draw more charge from the movement of the car than it would otherwise. Since the mass of the car won't have altered, that energy has to come from somewhere - and the fuel in the tank is that somewhere.

Unless you're driving a perpetual-motion machine, of course. And, in that case, how do you stop?

*prepares to be told he's wrong*
( Enzyme is powered by sunlight, Mon 27 Oct 2008, 15:45, Reply)

While the above is all true
the thing to remember is that the power to run the lights is a tiny fraction of the power output of the engine- so the difference in fuel economy between driving with your lights on and driving with them off is negligible at best. You affect your mileage far more by driving with the windows open (increasing the drag on your car) or with the air conditioning on, if you have it (increasing the load on the engine significantly) or by having soft tires (increasing drag on the road- try pedaling a bike with soft tires vs properly inflated ones to see for yourself) or by speeding (as you approach the terminal velocity of your car, more fuel is spent pushing air around).

So overall, driving with your lights on will do precisely doodly squat in terms of mileage. Or, to put it another way, it's on the order of how much mean sea level changes if you piss into the ocean.
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 16:13, Reply)

Thanks fellas, so...
...I'm wrong, but only a bit.

No air-con to worry about and won't be driving with the windows open for a few months now. Air pressure in tyres is on my list of things to do. Speeding is rarely a problem on the way to work and I've given it up to cut fuel costs, so, all in all, I'm pretty smug and content in my tightwadedness.

Will continue to burn those lights as I don't want to be another accident statistic on the A1079.
(Che Grimsdale Best before:, Mon 27 Oct 2008, 16:22, Reply)

Drag (friction, not sartorial eccentricity)
Further to the Loon's comments above, tyre pressues have a significant effect on how much fuel you burn. I've seen a noticeable difference between brands of tyre, some eco tyres save me 2-3mpg. That's far more energy than would be saved turning your headlights off.

Harder compound rubber in the tyres means they have less rolling resistence at the expense of outright grip. If you're not of the "drive it like you stole it" school of motoring, you won't notice any difference.

*edit* Can't see any fault with Enzyme's scientific theory!
(Bicycle Repairman "you're also a bit of a wanker", Mon 27 Oct 2008, 16:25, Reply)

Because this amused me
I did a quick look into it.

Let's assume that you're driving a car with a teeny little engine- say, 90 hp. That translates to 67,113 watts. Just for argument's sake, let's assume that you're only using 10 hp at cruising speed, which is about 7457 watts.

Now let's assume that your headlights are 60 watts each (no idea how much they really are, but that's probably pretty close). Let's assume that the rest of the little lights add up to another 80 watts, just for the sake of even numbers.

200/7457= 0.02682043, or 2.68% of your engine's output at expressway speeds, where you're only using 10 hp. Assuming 40 mpg, that's a difference of 1 mpg, lights on vs lights off.

Negligible. And if your car is more powerful, it's even less.
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 16:31, Reply)

Ok people...
I don't have a car, but I borrow one from work a lot.
It's a 3-litre automatic, awful MPG, somewhere around 29 if I drive it well.

How much difference will it make if I drive with the window open at 80 mph so I can smoke?

I assume "lots"?
( Cawl, Mon 27 Oct 2008, 16:47, Reply)

Kaol in drag shocker
According to my extensive calculations, about 77.5 Glimbarts could be saved by not using the car's cigarette lighter.

The moral of the story is bring your own bic.
(Bicycle Repairman "you're also a bit of a wanker", Mon 27 Oct 2008, 16:54, Reply)

Haha!
Mr. PJM, you've seen my Lighter Of Awesomeness, like I'd use anything else...
( Cawl, Mon 27 Oct 2008, 17:03, Reply)

That's harder to quantify.
I would assume that you're only opening it a couple of inches, so there wouldn't be that great an increase of drag- call it 2 mpg, maybe? But if you drive 60 mph instead of 80, you'll gain about 5-7 mpg or so. And at lesser speeds the open window's drag is less pronounced.

My Isuzu Amigo gets about 19 mpg if I drive it at 70 mph, and about 28 if I drive at 45-50. *shrug* And it has the aerodynamics of a concrete block.
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 17:04, Reply)

Isuzu Amigo = Vauxhall Frontera
Yep, aerodynamics and of course gearing.

Poorly executed gear ratios mean the engine works at less than optimal revs when running at your desired cruising speed.
(Bicycle Repairman "you're also a bit of a wanker", Mon 27 Oct 2008, 17:11, Reply)

Nah, I open the window all the way.
And crank up the stereo to compensate.
( Cawl, Mon 27 Oct 2008, 17:18, Reply)

On the other hand
the Amigo is a wonderful car to ride around in during a warm autumn day, as the back lifts off and it becomes a convertible. (And if I have a couple of large boxes in the back it improves the aerodynamics a fair bit, especially if I roll down the windows to let the air through.)

I can't really complain about the Amigo- I've put about 120K miles on it since I've owned it and it still runs.
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 17:25, Reply)

You people need to get out more.
*rolls eyes*
( girlinthehole, Mon 27 Oct 2008, 19:03, Reply)

I thought you had proper 4x4s (sorry, SUVs) in the States?!
The Frontera/Amigo is a hideous machine. Had to use one quite a bit this summer (by use I mean actually use off road, use the low box, tow heavy things in fields etc) and it sucked. Really really sucked. Would probably have been better off with a Rav4.

Trooper/Monterey, yes. Frontera/Amigo, no.
(Son of Sam-i-am We're all as mad as hatters here, Mon 27 Oct 2008, 19:19, Reply)

So does it use
a million times more petrol when I have the roof down?
( Mrs Entity is in hiding., Mon 27 Oct 2008, 19:26, Reply)

We do have 4x4s.
I didn't want an actual 4x4 at the time- I wanted something versatile that I could haul kids in. I've had it for eight years now, and it has served me well. My only complaint is that it's rear wheel drive, so it's not good in snow.

Mrs. L, it depends on the vehicle. If you're talking a convertible sedan, then yes, it does increase the drag- but that's only really an issue of you're going over 30 mph or so, in which case you're going to get windblown anyway, and probably you want that and don't really care about the drag. *grin*

Ever seen a pickup truck with a cover over the bed? It's not there to shelter the stuff in the bed- it's to keep the tailgate from acting like a parachute. Over here we have these plastic web gates you can put on the back that let the air through if you don't have a cover. It's actually illegal to drive with the tailgate down or missing, but you get a lot better mileage if you do.

Oh, and BGB? It's guys like us who think of these things who come up with things that save fuel and money and lead to better vehicles. You should be grateful that someone thinks these things through. *poke*
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 19:59, Reply)

@Loon
You can't poke me........I'm a lady.
( girlinthehole, Mon 27 Oct 2008, 20:09, Reply)

That depends
on what I'm poking you with.

My hands were on your shoulders at the time.
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 20:16, Reply)

Ah! I wondered why it wasn't a very strong poke.
.
( girlinthehole, Mon 27 Oct 2008, 20:28, Reply)

Arrrr!
I speared ye without using me harpoon!

And I can open a car without taking me hands out of me pockets, too!
( The Resident Loon Not a demographic. Do not measure., Mon 27 Oct 2008, 20:54, Reply)

19mpg?
that's not very much

I tend to get 55-60 at 80 mph in my Seat.

and that's with climate control switched on.
( Vipros. clever got me this far, then tricky got me in, Tue 28 Oct 2008, 8:56, Reply)

Fuel
My old Golf 16v used to manage between 35-38mpg at a steady 60mph, but because it demanded being given some stick the average consumption was around 31mpg.

My old Alfa V6 didn't have a fuel consumption guage. I can only assume that most owners would commit suicide if one were fitted as it had a drink problem. It would do something like 19-25mpg. Yikes.

BTW: The Loon's Amigo seems thirsty because US gallons are smaller than British gallons.
(Bicycle Repairman "you're also a bit of a wanker", Tue 28 Oct 2008, 10:21, Reply)

Good god, you've got UK gallons?!?
AAAARRRGGHH!!!! Every time I think I've gotten it figured out, you lot have to throw in a new twist! Why can't you just have gallons? Why do you need your very own special gallons?!?

GO METRIC FOR THE LOVE OF GOD! *brain asplodes*

19 mpg US= 23 mpg UK.

28 mpg US= 34 mpg UK.

40 mpg US= 48 mpg UK.

53 mpg US= 64 mpg UK.

Okay then: the Amigo gets, in UK units, 23 if I drive hard and 34 if I go easy. My Jetta diesel gets 48 in town and 64 on the highway.
( The Resident Loon Not a demographic. Do not measure., Tue 28 Oct 2008, 14:14, Reply)

I've no idea how many mpg
my ancient Toyota Carina E does.

I just fill the tank (c.50 litres), re-set the mile-ometer and drive. It does around 500 miles before I have to re-fill it in a good week and half or so. I've virtually 600 miles once which was pretty good I think.

So - what's my mpg?

[will have to start a regular car question here - it does wind up the ladies - hee hee]
(Che Grimsdale Best before:, Tue 28 Oct 2008, 14:56, Reply)

You get
about 45 mpg UK. Not bad- that would be in the upper 30s US.

Wait, do you use actual miles, though? Or are they UK miles that are 19/24 of a US mile or something like that?
( The Resident Loon Not a demographic. Do not measure., Tue 28 Oct 2008, 15:08, Reply)

Ahaha!
Loon, you just made me cackle :)
( Noeli overtheshoulderboulderholderthingstraplatchboobs, Tue 28 Oct 2008, 16:37, Reply)

Well
according to convert.exe, a UK pint= 1.2 US pints. So how do I know that a UK mile is anything like what I know? Here we have 5280 feet in a mile, but over there it might be 5947 feet 3 inches, because King Badgerpuss II decreed it to be the distance between his throne and his mistress's bedroom or something.

I mean, ffs, UK gallons?!? Is this in retaliation for throwing all that tea into the harbor or something?
( The Resident Loon Not a demographic. Do not measure., Tue 28 Oct 2008, 17:27, Reply)