Do lights have any effect on energy used?

That sounds about right, and even perhaps a less of a mileage drop. Assume a Prius is using 260 watts per mile, your average speed is 40mph, and the lights use a total of 150 watts (headlights and running lights). So 260 multiplied by 40 equals about 10,000 watts used each hour to move the Prius. Therefore the 150 extra watts used by the lights amounts to about 1.5% more energy usage.

BTW, you will most likely get better mileage at night because your A/C will work less, winds are likely to be calmer, and their may be less traffic allowing for less stop and go.

Rick
#4 2006

 

That is what I used (250 W/mile), so my calcs should be on the money. 60 mph average is about the best one can assume. Realistically, with typical mixed driving the average is probably more around 30 mph (~0.8 mpg). As Dave noted, slow commutes take a lot longer.

Problem is you are already minimizing your speed into a comfort/safety zone (as are most of us).

A better trade off might be to try to run with slightly less comfortable AC settings to hold the same average load on the electrical system. Time or AC are both comfort factors after all. So assuming you've already minimized speed but feel daytime lights are a must, there is not much else left to go after.

 

I didn't think it would make much difference (based on energy calculations) but in practice I found it had more impact on fuel consumption than I expected. For a while I was *only* driving at night and I swear I couldn't do better than 5.6L per 100k (42 MPG US). When driving mostly in the daytime I'm getting 5.0L per 100k (47 MPG US) and I can't think of anything that's different except for the power drawn by the headlights. (this was for around town driving with a 2004 gen II prius)

My theory is that the Prius gets alot of it's fuel savings because it can cut off the (gas) engine for a significant amount of the time (especially in "around town" driving). What I noticed is that the engine just stayed on a whole lot more often when driving with the lights on. Maybe it would have less effect on hiway driving when the engine is going to run pretty much all the time anyway.

 

Was there much temp diff between the day and night? Colder temps will effect mpg.

 

That is really high consumption in both cases, I'd be looking at the driver's style rather than headlight use. I suspect temperature is a factor or traffic.

I don't notice any difference driving lights on or off but traffic flows make a difference. I never noticed my Prius engine runs more with the lights on. I often drive with headlights on in the day time.

 

No the temperature is mild to warm here at the moment, probably about 22C in the day and only a little cooler at night.

Hi Pat, fellow Aussie here (central coast NSW). I see from your profile that you're getting 4.4L/100k on a 2004 Prius, that's pretty good - maybe I can get some tips. .

I haven't had the prius very long. Picked it up second hand just recently. The first few drives were at night and I only got 5.7 L/100k. I was really disappointed because I was hoping for a figure more around 4.5 to 4.9 given that I was driving pretty easy.

Anyway I reset it and driving *really* easy and slow around town the last few days I’ve managed to get it down to about 5.1 L/ 100km (honestly I'm driving the thing as if it had no brakes!). I really don't think I could go any easier on it without being a major nuisance to traffic. So yeah I'm a bit disappointed so far, maybe I got a dud.

The car seems to be in really good condition, only 70,000 km, one owner, full log books and 3 year dealer warranty (the previous owner just traded it for a 2010 model). Maybe it needs a tune up or something. Do you have any tips for things I should check to make sure it's in good shape?

 

Tyres pumped up to 50 psi front, 48 psi rear and accelerate harder, don't baby it so much. Keep big following distance, remember the 2 second rule and look ahead. That' is it in a nutshell. Remember terrain has a big influence, I have one good size hill each way but Adelaide is pretty flat over all. I also have a 1 hour each way commute which helps to get good numbers.

So don't worry about the head lights, that is piss ant stuff, the big influence is directly to the rear of the steering wheel.

Spend a little time in the mileage threads for more and better tips. Pick a style that suits you as it is hard to drive like a nanna when you're a petrol head like me. I personally accelerate hard to get to speed, then back right off and maintain speed with minimum throttle. It is amazing how much you can lift your foot without losing speed. I suspect I shock a few people who change lanes to get out from behind me as I coast to red traffic lights but then bolt off when the light goes green!

Oh, I more often get 4.3L/100km from a tank but as I sometimes got only 4.4 I thought I better stick with that.

 

your experience is the exact opposite of mine. at night with the lights on, driving the same route, my mileage went UP. reason; no traffic. averaged 2-3 mpg higher (this was daily commute for 2 weeks when we expanded. spent the time closing an off-site training facility and moving all the computers to the newly expanded main building)

and the traffic i only encountered on half my daytime commute. i work early enough in the morning (6 AM) that i miss the morning rush hour. but working 10 hour shifts means that getting off at 5 PM, means i am smack dab in the middle of the rush going home.

now, i have to make a very important disclaimer here. the mileage figures i state are for the 2004 tidelands and the 2006 SPM (no difference in mileage between the two) both have limitations on regen the 2010 does not have and on the commute home that made a huge difference since it was not "stop and go" it was "crawl and park" commuting.

now since getting my 2010 in mid May, i have used it to commute less than half a dozen times. so no data on it. i might try it just for comparison purposes, but its against my principle to burn gas when a much better option is available, (plus i am a cheap bastard and i get free juice from work!!)

now i do have plans to test the 2010's commute mileage but not until winter. although it does warm up much quicker without the thermos, it still takes longer than i want it to especially on my short slow commute. so will test with and without grill block probably a week or two without (or as long as i can stand the low mileage!!)

 

Oh I will mention here, I have blocked my upper grill but it had no affect on mileage that I can detect. I'm going to take it out this week, we are expecting 27C over the next few days and I am seeing engine coolant temps around 90C now.

 

Pat; grill blocking improvements drop dramatically as the commute lengthens. my commute is 7 miles one way, speeds 25 MPH for 10%, 35 MPH for 40% and 40 MPH for 50% of the drive.

so what we are looking at is mileage when car is cold and mileage when car is warm which is better. the shorter the commute, the larger the percentage of cold car commuting mileage.

now, since you are down under, you need to take your monitor and turn it upside down for any of this to apply to you

 

Ok so after reading what others have said I now think that the main difference between my daytime and night time measurements were probably due to different terrain. There were some quite large hills in my night time route while my daytime trips have been on somewhat flatter terrain.

I should also mention that I haven't put *any* fuel in the tank yet as the dealer had half a tank full in it when I picked it up. I'll have to fill it tomorrow and I'll probably just use regular unleaded.

I'm thinking that if the dealer put E10 unleaded in there (likely since it's cheaper) then just going to regular unleaded might improve the mileage - I've seen some valid looking tests that showed E10 typically gives 4 to 5 percent less mileage (but is only 2 to 2.5 percent cheaper so not really a good buy). Actually I'm hoping that the petrol in the tank at the moment is just cat's piss and ethanol - so then I might get some real improvement when I fill up

BTW. I'm using 40 PSI all round at the moment but after reading some recommendations here I'll go to either 40/38 or 42/40 next time I’m checking the air.

 

ethanol has about 10% less power so overall mileage should only be 2% less. terrain is biggest effect on mileage simply because you cannot do anything about it. (well can a little, but gravity has no friends)

 

Yeah that's probably about right. The first "study" I found on the net found about a 4% difference but the more I look the more different opinions/figures I get for this. It seems that most of the studies I can find put the figure at between 1% and 4% and apparently it's different for different vehicles and different driving conditions. I suppose I'll just have to test it myself if I want to know for sure.

 

From what I've seen the net impact of ethanol is much greater than 2%. 5-10% is what I've observed in other vehicles. About 3% would be the lower bound.

This 3% loss is coming from the motive portion of the power production from what I can tell, meaning its impact on actual mileage is multiplied.

Oxygenates in gasoline have never been an intelligent concept.

 

ethanol is only 10% by volume, if the overall loss of energy was much greater than 2%, the ethanol would contribute about the same as water.

 

Hi Dave, this is the report that I was originally getting my info from :
False economy: fuel types compared - www.drive.com.au

They compared the fuel consumption of 3 identical Camerys over 2000km and found about a 4.3% increase in fuel consumption for E10 as compared with regular unleaded ( 9.41L/100km for the regular unleaded versus 9.81 litres for the E10 fuel). Bear in mind that although this test appears to be done in a somewhat controlled manner it was done by motoring journalists and not by scientists or engineers. So I wouldn't be surprised if their methodology is not really perfect. (Remember that journalists have a pre-disposition to obtain a "controversial" result).

 

If you hyper-mile, yes, headlights effect your mpg's. Not so much regular driving ... but in a hyper-mile state, the largest parasitic draw is likely to be headlights. I do better on my drive home in the daylight while hyper-mileing than I do on my drive home, because it's always dark when leave in the morning & get to work before 5am.

.

 

an increase would be unusual

 

An increase in fuel consumption, that's a decrease in MPG.

 

ooops!!

ok, well i have only done regular gas a few times (none in the past 4 years) since E-10 is the only option where i live so hard to say what my mileage hit is. my signature tells you what i get.