Sudden Significant Drop in MPG

welcome! ouch, that's a big drop. has anything changed at all? commute? tyres? tyres pressure? oil level, recent service?
one thing, there is a switch to winter gas blend sometime around now, which can be a 10% hit to mpg's.
make sure your parking brake is free. put it in neutral at around 20 mph to see if she free wheels okay. be careful, there's a sudden surge when putting in neutral, do it away from traffic.

 

Here is the main questionnaire that helps us in troubleshooting your MPG, though it doesn't include brake drag or check engine lights:
Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new | PriusChat

For new owners, see also these threads:
New owner? Want MPG help? Read this first. | PriusChat
Why Mileage Gets Worse in the Winter | PriusChat

 

Has it been colder than normal for a week or more? Colder temps affect MPGs. Winter=worse MPG. Summer=better MPG.

Maybe your 12V is drained, causing the engine to run more often than necessary? Perhaps a trunk, map, overhead light was left on accidently, causing this drain on the 12V?

 

It's always possible that you left the car on overnight or for an extended period of time. I often camp in my Prius, and when I do mileage drops from mid 50's to the 30's, depending on the temperature as I leave the car in ready and the climate control in the 70s. The ICE starts occasionally to keep the traction battery charged, consuming fuel.

 

Lol mine dropped like a roller coaster after maintaining 58ish MPG per fill up. My engine stays on longer as it be like 40 degrees in the morning and 50-60 degrees during the day and back to the 40s at night. Put 5.8 gallons in and nearly at a quarter tank with 190 miles I driven. That's like 34 MPG by hand but my MFD reads 46 MPG. I gotta get some engine block heaters.

Posted via the PriusChat mobile app.

 

Be sure to buy it from PriusChat. Engine Block Heater for 2004-2009 Toyota Prius - PriusChat Shop

A timer to turn on/off the block heater so only enough electricity is used. Maybe 1-2 hours is enough before you leave in the morning. Otherwise, you'll plug-in when you arrive (say 6PM), and unplug when you leave (say 8AM); 14 hours of electricity, vs just what you need (maybe 2hrs).

Be sure the timer has a high enough amp rating to support the block heater.

 

Wow...ours according to the since inception dash computer remains about 41 mpg.
Lots of variables.
Here's some ideas:
a) How is the OP deducing the mpg dropped?
b) Winter is engine running more of the time?
c) Change in fuel composition or brands?
d) More acceleration, different travel routes, other drivers?
e) Tire pressure un-altered?
f) Fuel leak....smell any fuel under the hood?
g) More weight/ luggage/ passengers?
h) Brake drag? Feel a wheel after a drive; is it hot to the touch?

Common reasons in my experience: Tire pressure, more uphills/ acceleration, winter fuel blends w/ E85.
20% drop is fairly strong drop....not sure if that is in a forum member experience A-O-K range.

Let us know what you discover.

 

have you put a lot of weight on recently?

 

Oh oh... you gotta be careful with statements like that...

 

them springs going down

Posted via the PriusChat mobile app.

 

If your variation is based on one tank of gas, the shutoff on the pump nozzle may vary from time to time. I often have wild swings in MPG, low followed by high or the reverse but the average over many fills is remarkably steady.

 

can sometimes be a thyroid thing.

 

Around that time of year, depending upon where you live, gas stations will switch to "winter blend" gasoline. This has less energy per unit and as such, you will get worse gas mileage with it. Further, if you use the heat a lot, that will wreck your mileage. Driving the same way all year (except for when I have to drive in snow), I get about 46 mpg in the summer and 40 in the winter.

 

Ah, a wonderful time in Michigan - done with winter blend, warmer temps but not so warm as to demand A/C. Not hard to see 50 on the trip computer - which means 46-48 in real life.

 

what happen to the o/p, was it something i said?

 

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.

 

Over what timeframe did this happen? If summer until this month, it could be just the natural seasonal MPG change.

A single 10 mile trip is much too short to be certain that the battery replacement changed anything. More trips over multiple days and weather / traffic / warm-cold-start conditions need to thrown into the average MPG pot.

Why mileage gets worse in winter | PriusChat
Why Mileage Gets Worse in the Winter | PriusChat

 

yes my prius prime dropped from a highway mpg of 69 to 48 overnite . have no idea why taking it to the dealership soon.

 

What was the weather when mpg changed?