Lights on = lower mpg?

I'm very new to the whole Prius scene and we only have about 500 miles on ours but during a 2 hour freeway trip in the dark and rain we got 50 mpg. That seems good to me for driving 65 MPH with three adults and the heat set at 75 in the car but I'm curious as to how much better it would have been on a nice mild day.

 

Hi DBaum,

I'm sure the experts will chime in, but I believe things like the lights are powered by the 12V battery, not the "big battery". Running the heater will definitely affect mpg though...

I've noticed on my IV with heated leather seats, I can turn the heated seats on and the engine will not run, but the second I hit the heater switch (with or without heated seats on) the engine will fire up.

Maybe you should take note of your girlfriends driving techniques and try some of them?

 

I have found that if I use the front fog lights the big battery appears to discharge much quicker than if I just use normal lights causing lower fuel economy. Can't imagine that two extra bulbs makes much difference but my experience suggests it does. I also find that very wet roads after/during heavy rain can also severely affect fuel economy.

Anyone else found this?

 

It's been raining like crazy for the last week here (standing water in streets very common). Before this week I was averaging ~48 MPG. This week, I'm averaging 44.5, or a 7% decrease.

I attribute it to increased rolling resistance due to water on streets, and increased battery use due to headlights and wipers.

 

I think it has to do with the way you drive, nothing to do with your lights being on.

 

in the Pri, anything that uses power will ultimately affect mileage so ya, the lights on will lower mileage.

look at a sprinter and a walker, both carrying a young child (well used analogy i know, but applicable). now whose perfomance is most affected? the sprinter of course, but he normally (when sprinting) is performing at or near his peak. the walker is not.

so look at the Pri as a sprinter, the walker as a gas car. the gas car uses the alternator to provide the power to the lights. the alternator, most of the time, puts out more power than the car actually needs. granted its metered so during lower power needs, it has less drag on engine, etc... but point is, it has potential to provide a LOT more power with a relatively minor effect on engine performance.

the Pri is just the opposite. it, by design, is set up to run as efficiently as possible. the problem; converting from high to low voltage is not efficient. so when the lights are on, more conversion, more loss, etc. is noticable since the Pri wastes less energy to begin with....

ok, now after all that...

many here have tried to measure the drop in mileage as have i, and i dont know of anyone here who has a measurable figure to give you.

i work 10 hour shifts and live north of most of the US. so i go from daylight both ways (about 3 weeks a year) to dark both ways ( during time change) to dark in morning only (most of the year) so...

i have attempted it 3-4 times and i have a lot of data but none of it really shows a distinct difference.

 

Any power consumption will decrease MPG, but the effect of lights should be tiny. The day vs. night temperature difference ought to have a larger impact. And differences in driving conditions and style will swamp even that.

I leave my lights on fulltime.

 

Umm, guys, I know we want the best gas mileage but don't start driving with your lights off if you need them! Even in twilight, turn on your lights.

 

In ready mode, the DC to DC converter is pulling amps out of the HV to charge the 12V. All the energy stored in the HV and the 12V came from the fuel tank. Thus, every electrical device is powered from the fuel tank.

Biggest some-of-the-time drains are heat-related things like AC and defrost. Less true of the heater if you are on long highway drive, as the car produces excess waste heat at those speeds whether your cabin is warm or not. But in town, the engine heat going to the cabin is observably running the ICE more.

Light bulbs (esp. non-LED) produce a lot of waste heat, so it is a factor, but not at the level of AC or defrost by any means. I doubt the extra cost/complexity of LED headlamps (which come on the Prius with washers and levelers out of respect for on-coming drivers) would pay for itself in fuel savings. If we converted all incandescent and cfl bulbs in our homes to LED, we would clearly see the savings in fuel cost, but the big appliances would keep a nice fat monthly check going to the utility.

Driving style and speed, temperature, rolling resistance, etc. swamp the impact of the headlights. Add a couple of psi to the tires and put headlight power in the column you cannot do anything about.

 

On a personal note: I would be much more saddened by the news you were in an accident because the 'other guy' did not notice you, than I would be happy to hear you saved 0.3 MPG by leaving your headlights off. Run your headlights all the time.

 

I respectfully disagree. I oppose DRLs, too. But one should certainly run headlights in any reduced visibility environment, and as required by law. If we are unable to see one another in broad daylight, then the speed limit must be too high.

 

Ah, I was raised in a 'reduced visibility environment' without 'broad daylight' for 9 months a year, on Grays Harbor in western Washington. The ability of see headlights through the trees can not be over rated.

Aberdeen, Washington - Wikipedia, the free encyclopedia

My mother claimed she was 18 before she found out Grays Harbor is named for a man not the sky.

 

At night, when I need to see the road, I keep the headlights on (of course). But when it's simply cloudy/overcast, etc, so as to be seen, I run the much, much lower wattage parking lights - only. They're pretty bright, and I think (?) LEDs, which use very little juice - well more than an order of magnitude lower current than used by the headlights.. In light rain, or mist, I use parking lights. Hard/steady rain, I use my headlights. Common sense should apply...

 

Do us all a favor and turn your headlights on when its raining. Even parking lights get obscured, just look in your rear view mirror when its daylight and raining. Hard to see other cars, especially when other cars behind have their headlights on.

 

If this comment was meant for me, then - As I said, if it's raining steadily, the headlights come on. Mist, not so much. Biggest issue with lights being obscurred is the amount of water coming off the cars around you. The parking lights on the Prius are pretty bright, and I have a white car....not concerned with other cars not seeing me in misty conditions. Besides, most of my driving is on highway, or other divided roads. As a Prius driver, who's trying to conserve fuel, I have a far GREATER risk from the truck behind me coming up on my tail...and to tell you the truth, that's why I put the parking lights on. Not worried about the guy in front of me.

 

I don't get it, you want to save a miniscule amount of fuel by not running headlights, yet you bought the heavy sunroof option?

I experience this all the time, some guy not running his lights when its "misty" out because he can see out. Then if you try and change lanes, its hard to see a car there. Just turn on the headlights, its safer.

Why do you think they are called parking lights? Its for when you're stationary. Just my opinion of course.

 

The headlights/wipers on is a law in Delaware also. But, I see many cars that run around with their lights off in a downpour. I believe I read somewhere that tests have proven that others can see you better with headlights on regardless of the weather. Your headlights will be seen way earlier than the actual car itself will be. To put the headlight power draw into perspective, they pull 55 Watts. I believe that's 55 Watts in an hour. I'm not too sure how powerful the HV battery is, but even 55 watts is next to nothing. The biggest hit to MPG is the heater, A/C, and weather. I was down around 42 MPG a couple tanks ago. Luckily, temps have gone back up to the 40s and I'm back around 45 MPG. I've also been driving without the heater on for the short trips.

I put some Blizzaks on the car recently with new alloy wheels, and am curious as to whether they are affecting MPG any. They actually seem to glide longer than the OEM ones did, and they are a LOT quieter! Ok, I know that was off topic, though!