MPG decreases from 52 to 43? PLEASE HELP!

I'll take a stab at this, though I only have an outdated Gen II.

I would suspect some kind of short in the fog light wiring. The short
would continuously discharge the smallish 12V battery. This is turn
would be continuously recharged from the HV (High Voltage) battery
via the DC-to-DC converter. The HV battery would be recharged by
excess running of the ICE (Internal Combustion Engine). This would
show up as continuing to run for a while when stopped, even though
the car is fully warmed up.

If you had a ScanGuage, you might also notice when driving around
that the ICE is running at higher than normal RPMs as power is diverted
through the PSD (Power Split Device) from propelling the car to turning
MG1 Motor/Generator1) to keep the HV battery charged.

Let's wait and see what others have offer.

 

I was going to suggest that the MPG indicator had been reset when you unplugged the 12v battery. I hope you unplugged the 12v battery before making electrical modifications to the car. If the system had reset then your MPG may change until the car relearns your driving style and conditions.

If your battery was never unplugged or triggered to reset then I am leaning towards what Rokeby posted above.

 

Here are just a couple of many items that will need to be explored:
(1) How many miles have you driven since installing the fog lights?
(2) Did the weather change?

 

omg, this is a modification consequence at its worse

I have no ideas but others above are much more techie than I am. Good luck.

 

Very Interesting.....

I have experienced something similar, but it did not coincide with installing fog-lights... which I did also (purchased here on PC).

I have a Prius IV with just over 10K since May 31st. I had left the trip gauge running and had averaged 50.3mpg for ~8,000 miles. Then the last 2k miles my mpg has dropped considerably (average down to 48.6). I just took it in to the dealer for its 10k service and mentioned the problem to the service writer. He had the computer system checked out and reported that everything was set normally.

We have not changed the area we drive or the style of driving..., but the miles per gallon have definitely dropped.

 

How much has the average temperature dropped since you noticed the change in MPG? At 10K miles I am assuming this is your first winter with the car. Winter weather will drop your MPG.

 

I had a similar thing happen a few times...

1. Wife driving the car
2. Winter gas (yes it affects your MPG)
3. Road terrain
4. More liquid on the road (in So Cal, during our 2 months of rain, you get considerably poorer mileage)

It has nothing to do with your mod.

Average out your tanks over a month (no spikes in data). I get 44 MPG always - but I live in a hilly area...

 

Definitely something else wrong here. I saw zero mpg difference after I installed my fogs a little over a month ago. There must be a short somewhere if you are getting the check hybrid warning.

 

I noticed my MPG dropping a lot, checked my tire pressure, it was like at 25! not low enough to trip the warning light, but low enough to seriously affect gas mileage. we need to check our air pressure at least once a month, maybe twice.

 

Interesting problem.....
Disconnect the fog lights at the fuse panel for a tank and see if there is a corresponding change in your MPG.
I would expect that your mileage would vary with weather...(even in the PRC) and other factors but not by 20-percent...unless you've discovered the joys of driving a sports car, or have recently let a teenage driver loose in your Prius. It sounds more like there's something amiss in your electrical system---especially with the Check Hybrid System indicator.

Please let us know if you find something.

Good Luck!

 

Mine has also dropped and I drive the same roads daily since I got my Prius 1 year ago. What's going on here? Most cars get better mpg as they break in, not the Prius?

 

We cannot answer your question without you giving us specifics on the dates your are comparing your MPG to, the weather conditions you were experiencing during these times, any maintenance you have performed on the car, etc. There are way too many factors involved with measuring MPG so one cannot just assume that your car is getting better or worse gas mileage only because of break-in. If you are looking for a real answer please click on the following link and provide answers to the listed questions.

http://priuschat.com/forums/gen-iii...-answer-these-questions-esp-if-youre-new.html

 

Bought the Prius in Nov 2009 and I was getting form 49 -51 mpg up until June of 2010 when it started to drop. Through the summer is was from 47 - 49 mpg up until Oct 2010. From Nov to present it now from 41 to 45 mpg.
I travel the same route / roads every day. The only mods I made were (new audio system in March 2010, mud flaps in April 2010 & tinted windows in June 2010. Approx April 2010 was my first oil change.

Any ideas?

 

My 2010 has 8000 miles.It has dropped to 31 mpg and 2 dealers says theres nothing wrong with the car even though it use to get 48.6 on the same trips 4 months ago. They also told me theres a bladder in the gas tank that is effected by barometric pressure and so I shouldn't pay attention to mpg. Called toyota we care three times and they don't get back to me. I'm wondering if toyota is paying dealers not to fix cars that are under warranty.

 

No bladder in the 2010 gas tank.

 

There is an issue here, the Prius is a nice car if getting the mpg they state but I would of never bought the Prius if it got these mpg. Hybrids are not selling well in the US, EV's are not practicable and there is no real solution yet. Maybe there is but like everything else "it's all about the $$$" and the big companies are ripping us all off.

 

It's always about the $$$. Always has been.

Hybrids (of all types) actually are starting to sell well in the US, since gas prices have slowly crept up from 2.something a gallon to 3.something for the same gallon. This is a process that will probably accelerate as cash-strapped state governments start looking lustfully at increasing their gasoline taxes.

I'm not exactly a rabid cheerleader of Priuses, but in the nearly 3,000 miles of stick time that I've logged with this car, I've come to appreciate that it does get the mileage that it's rated to provide---and a base model Prius is not too bad a choice economically if you want to get a car for commuting.

My II currently indicates that it's getting 55.x MPG on my current tank according to the 'dizzy-whizz' display (394 miles on this tank, 39mph average speed.) This tells me that I'm really knocking down about 50-52.
Just about what it's supposed to.

If your mileages are dropping it's probably due to a number of factors including designer gas blends, lower tire pressures, increased heater usage, ICE warm-up periods, throttle [sic] use, etc...
Yes, these factors will influence the fuel efficiency of a Prius more than they will a GMC Sierra, (mine gets 21.5 summer and winter) but I don't think that Toyota Iron Works has hatched an evil plot to enrich their bottom line by depriving you of a few points in your fuel efficiency spreadsheets.

FOR RIGHT NOW...the Prius is about the best bang for your buck in terms of fuel economy and cost of ownership, and while I'm constantly disappointed in this car's ergonomics, and some of the engineering decisions that went into the G3---you cannot say that it's a "bad" car, otherwise Toyota wouldn't be selling nearly as many of them as they currently are.
The good news is that Priuses have a pretty high resale value. If you don't like yours then you can always sell it and get a 'better' car...whatever that is for you personally.

 

As I stated in my above post - I bought the Prius last Nov 2009 and drove it through one of the worst winters and avg 48 - 52 mpg. I drive the same exact roads now and only get 41 mpg. My tire pressure is the same as it always been, and I use the same gas since new. There is something going on here and my Prius is not the only one. You will have the same issue when you reach approx 7,000 miles. Let us know what your mpg is at 7,000 miles and you will join the dropping mpg club.

 

+1. It might be a good idea to make sure your getting 0w20 synthetic oil if you take it in for servicing. Dino oil would really be a problem in cold temps and play havoc on mileage.