Running with Lights On in Cold Weather

Your assumption is incorrect. Headlamp current draw may vary a bit with temperature, but no where near what you are reporting. The amount of power used for the headlamps is pretty insignificant, so you really shouldn't see a measurable mileage change from using them. Look elsewhere to explain the poor mileage. Bad gas is a possibility. It's also possible that you got a partial fill.

Tom

 

Yeah, that's 100% bogus. Headlights use far too little energy to impact your gas mileage in the winter. The single biggest killer at this time of year is temperature, followed by our efforts to be warm in the midst of said low temperature. If I really want to get better mileage around town I just don't bother to turn on the heat, and the engine doesn't have to run as often just to keep itself (and hence me) warm. You have to defrost, of course, but that's just how it goes.

The other thing is that you don't say how you calculated your mileage. If you use the miles divided by gallons method then you really have no idea...lots of threads explaining that.

 

The culprit is in the 2-10 mile trips in cold weather I think. I am having the same issue. MPG has been getting more & more dismal the last 3 tanks. Since it's been snowing every second or third day, then freezing up by the next morning, I have had to de-ice and de-fog the car far more than usual. On top of that I am only going to work & back and a few local places. The result? 25 mpg on the current tank. It is so discouraging that I didn't even look at it when I got home today. Only two trips over ten miles on this tank - both times in snow/sleet with heat, A/C, wipers, headlights and defrosters on. Add to that all our gas has 10% Ethanol and it's a lose/lose situation. And I am parking in the front spot in my yard so I can get out before the whole driveway is shoveled, so can't plug in my EBH. Can't wait for spring!

 

On a Prius, the only thing the 12V system does is boot the computers and close the relay for the HV system. After that all the power comes either directly from the HV system or through the DC to DC converter to the 12V bus. Even then, the only things drawing from the 12V bus are the lights, computers, and some accessories. It's not like a normal car where the 12V battery has to crank the starter. I don't see the temperature making much difference in this area.

Slushy roads are a different story. They really kill mileage. The big culprits are short trips; cold weather; soft, snowy, or slushy roads; low tire inflation; and poor tires. Winter supplies several of these conditions.

Tom

 

In cold weather the big culprit is the ICE runs longer to heat the radiator to supply heat for the cabin. try blocking some of the vents. There are many posts at PriusChat that talk about that. My mileage drops from the low 40's to the high 30's when the temperature drops to the 30's and lower.

 

I decided to try driving with the headlights off whenever possible....

Please only do this when it is "safe" to drive without your headlights on.

In CA, a law was passed that you have to have your headlights on if you are using your windshield wipers. Basically, the assumption is that if it is raining, it is dark enough to warrant headlight usage. I'm assuming that the same would be true if it were snowing.

I suffer more than others when I have to use my headlights, as my Integra has the "pop up" headlights and it really messes up the aerodynamics of the car (far more than the additional draw of current would). But then again, my car isn't a hybrid.....

 

One of the things that your Prius has to do that requires the ICE, is heat up the exhaust system to lower the emissions. In cold weather, it takes longer for the exhaust system to reach the optimum temperature, so the ICE runs longer. The longer the ICE runs, the lower the MPG.

Keep the perspective of how old-fashioned vehicles also have lower MPG when driving through bad weather, slushy conditions, and with the heater/AC running at full blast, and notice how much BETTER the Prius' MPG is over those vehicles!

 

As discussed in many threads those short trips combined with lots of heater use is a super killer! When possible I don't turn my heater on until my temp gets to about 155 F or higher. As the heater draws the heat away from the engine, the core temperature start to drop. During the early stages of a short trip, when I come to a stop, I monitor the temperature and turn the heater off when the temp gets down to about 148-150 F...when the engine temp falls a little below this, it will cause the ICE to kick back on even if you are at a dead stop....this will wreck your MPG. When I start to move again which triggers the ICE back on and increases the temperature I throw the heat back on.

(Some funny things I always wonder about a) How many of us freeze in our Priuses just to better our MPG a bit? b) How many of us cringe when the wife takes the Prius on the weekend to do errands during the winter...cause you know she's got the heater just blasting away....the first thing I do when I get the car back is to see how much she has massacred the MPG!...[I don't mean to be sexist here or offend anyone, so please don't write back and tell me what a jerk I am...Sorry honey, I love ya!]

 

I agree...short trips will display a dismal low mpg. This morning, my car displayed 22 mpg average on the first 10 minutes. After riding for about 15 miles, my mpg jumped to 48 mpg average... I now have learned my lesson. If I just have to run some errands,,,say to a grocery store...it is just as beneficial to use my other car.

BTW, I run with my HID lamps 'ON' all the time. I does not affect my good mpg at all.

Drive safely.

 

Nice snowy afternoon, white car no lights,,,where, there! look out!!!

Rainy day, grey car, look out! where? There! look out!

It drives me crazy to see cars driving with no lights on, even in twilight. Even more crazy on rainy/snowy day! Next time take a look and see how hard it is to see these morons!

I drive with lights on almost all the time! Daytime running lights were the law in Canada for years, but I think they have canceled that.

Icarus

 

I agree 100 percent, and will add this: when snow covers the ground, your eyes have to close up (pupils contract) to limit the amount of light. This tends to make the road and any vehicles on it look darker. For that reason, and also because many vehicles will be covered with dark road grime in the winter, daytime headlights are even more valuable in the winter than summer for safety reasons. Making yourself visible to other motorists is square one in driving safety.

Harry