Sudden drop in fuel economy

It could, but they would have to have driven it a lot for it to make a difference. If the friend manage to change the tank average significantly while driving it, then maybe.

The battery could fail in a way where it significantly drops the MPG without setting a code initially. If that is what happened, expect a battery code soon.

Could it possibly be bad gas, or high ethanol gas?

Have you checked you tire pressures lately? Other things to check is the oil level, is it significantly over filled? If it is drain off the excess oil and then check and clean the throttle body and the mass air flow (MAF) sensor.

 

I would check the 12v. battery especially if it's the original. One sign of the 12v. battery going bad is the gas engine running extra to keep it charged. Since your mileage drop happened so suddenly maybe your last start was the one to start the downhill slide of your 12v. battery.

 

While it would be a good idea to check the 12 V battery, in any case, I doubt it is the cause of the ICE (internal combustion engine) running frequently or constantly. For this to happen the HV battery wouldl have to be discharged from 6 bars to 2 bars first then the ICE will kick on frequently. For the 12 V battery to even be able to do this, it would need to be seriously bad shape. It is more likely to be the A/C set at 74ºF on a 90ºF+ day. It draws heavily on the HV battery. Even more so if the HV battery's capacity is diminished, which is possible on a 2007.

 

12v. Battery going bad makes the Prius do a lot of weird things LOL.

 

It may seem that way to people who do not understand what is going on behind the scenes.

But what you put forward is not even one of them. LOL

 

In that case, it looks more increasingly like something is going on in the HV battery, if you are ruling out those other things I mentioned.

 

Did you reset the fuel mileage tracker in the display to see if it still gets that mileage with just you driving to take your friends driving out of the calculation?

 

someone just got a great deal on an optima at pep boys.

 

Poly cos it's defective. LOL.

 

how low? i guess if they were 20 psi it would make sense.

 

I have found anything less that 33 PSI makes a noticeable difference. Anything over 38 PSI make little significant difference, all other things being equal. I don't believe that over-inflating* for the sake of a couple of extra MPGs is worth the trade off with tires wearing quicker down the centre (which will occur if tires are over-inflated*) which means they wear out early verses the saving in fuel cost. To me it is a false economy as the cost of new tires outweighs the saving in fuel. The best option is to select the best LRR (bearing in mind the premium for LLR) tires balancing the fuel economy and eco premium.

This is no surprise to people who have been around Prius for any significant time. We try and tell noobs all the time bit they don't believe it will much of a difference.

Remember though there are many little thing that do not, in and of themselves, add up to much, but collectively, they can be the difference between good and bad MPGs.

If it were any easy equation, we would have so many angst filled post of "which tire should I get for fuel economy".


* Bear in mind the definition of under-inflation, correct inflation and over-inflation is governed by tire tread wear pattern and is the one to which I refer. Correct inflation is shown by even wear right across the whole width of the tire and not the use an arbitrary PSI figure. The actual PSI will be different tire to tire by +/- ~3-4 PSI.

 

I have been dealing with mileage dropping over the last 4 months. At 100k, I decided to do some maintenance and with the help of NutzaboutBolts, I went ahead and performed:
1) transmission oil change
2)engine oil change
3) spark plug replacement
4) install oil catch can
5) removeand clean egr valve amd tube
6) remove and clean intake manifold
7) remove and clean throttle body
8) replaced O2 sensor
9) replaced MAF sensor

All of this was proactive maintenance. I had zero issues and no check engine lights.

After all of this, I experienced a slow decrease in MPG- from normal of 50-54 to 44-47.

No matter what I did, light on the accelerator, tires at 40psi, premium fuel, ECO Mode, and overall conscious driving g habits, I couldn't get this thing back to "normal."

Today I decided to bite the bullet and buy a new 12v battery. I had no reason to suspect this was the issue because I purchased this Prius used 2 years ago and when I looked at the battery 2 months ago, the sticker on it said 8/17. I figured the battery is 2 years old- still good right?

Last week, while sitting in my car watching my in-dash dvd player (which came with the vehicle) a message appeared that I had not seen in the past. I typically sit in my car for an hour or a little more, 3 days a week while my daughter has soccer practice. I'll usually pop a dvd in and watch it.

The message said something to the effect- shutting down to conserve battery. I am always in Accessory Mode when watching the dvd.

Today I swapped out the 12v battery with a new yellow top Optima at a cost of approximately $230 dollars at local AutoZone. Drove it right after on the highway for about 10 miles and I'm up over 54!!!

Hopefully, problem solved

 

Don't keep doing this, unless you want to be changing the 12 V battery regularly. If you must sit in the car using accessories, just leave it in READY and allow the HV battery to supply the 12 V bus via the DC/DC converter. The car will take care of keeping enough charge in the HV battery and the DC/DC converter will keep the 12 V battery charged up.