Why would my fuel efficiency decrease? weather or accident..

I would lean toward the weather. My Prius was averaging 51 mpg before it got cold here in DC, and now that it's cold, I'm had varying tanks from 43-48mpg. I've also noticed that slick roads lower my mileage.

One quick thing to check, make sure the air pressure in your tires didn't drop as well. Mine dropped about 5psi a tire when the cold set in, and once I refilled them, my gas mileage back up a bit. Which reminds me...I should take my own advice.

 

I live in the Philadelphia suburbs. With the warm weather I was averaging 48 - 51 miles per gallon. As soon as the very cold weather hit it dropped to 39 - 41 miles per gallon. Last Jan I was averaging 42-43 miles per gallon in the cold weather. So, if your mileage numbers are similar my money is on the cold weather.

Springtime

 

<div class='quotetop'>QUOTE(CJmebm @ Jan 30 2007, 04:09 AM) [snapback]382714[/snapback]</div>

Colder weather could definitely be a factor. The question is "What is the nature of your driving?"

If you're taking shorter trips the hit will be worse. If you can get back closer to your higher mileage by taking an extended trip (let's say 30 or more miles) on relatively level roadway, you may get a better idea.
Look at your 5 minute bars.

Dave M.

 

It is not necessary to double-post. Most PriusChat regulars will check several categories.

 

<div class='quotetop'>QUOTE(CJmebm @ Jan 30 2007, 07:09 AM) [snapback]382714[/snapback]</div>

Mine is a bit older than yours but the biggest drop in mpg I saw was ~10%. To maintain mpg I increased tire pressure to 42/40 and blocked the grill. If you can, keep the heater off for the 1st 5 minutes and set the temp to 70. What was your mpg in the summer and what is it now over the same route?

 

its mostly the weather. in the past week, we have only had temps in the high 40's and low 50's and i am getting 4 mpg higher than when temps were in the mid 30's two weeks ago. that is only a change of 15º. with a change of 20-40 like you mention, the difference will be even more.

one thing i noticed is mileage changes from 60 to 40º are noticable but nothing like the difference from 40 to 25º. when it gets below freezing and you start using the heat and defrost, that is when your mileage will plummet.

a few weeks ago when we had our cold snap, i was using Scangauge i borrowed to monitor water temps. my temps would be in the 180º and i would turn on the heat and water temps would drop to 155º. i would turn off heat, wait until water temps got back up to 180º and turn on heat and same thing happens. so in a Prius, its efficiency levels are so high that EVERYTHING affects the mileage.

if you are not blocking your front grill, i strongly recommend it. you will see a good 2-4 mpg increase

 

<div class='quotetop'>QUOTE(CJmebm @ Jan 30 2007, 04:09 AM) [snapback]382714[/snapback]</div>

it is noted that when the battery is disconnected that the computer goes thru a learning process of sorts that can have an effect on the mileage and engine perfomance.