Sudden 25% drop in MPG

Just to clarify: are you in the United States of America? Hence "PA" equals Pennsylvania? And those are Fahrenheit degrees?

How many miles on the odometer? New tires recently? What speeds are you doing on the highway? Are the roads clear, or some snow?

Just my 2 cents: 50 MPG in sub-zero is not bad.

 

i would check the 12 volt, but the rough engine might be bad gas, throttle body, plugs, etc.

 

For daily driving, 12.2 volts is pretty low.

Might be also build-up of tar/gunk in the Exhaust Gas Recirculation circuit, though with your miles seems a little soon. The canary-in-coal-mine could be the inside of this pipe. You need to take the air cleaner housing off to get at it, and the bolts at the nearest-to-you end are obscured by some plastic clips you have to remove for access. The clips are hard to release, but it's possible. You can just see the elbow of the pipe in this pic:

 

Looks like the maintenance lost my posting, about 4 hours back. Anyway, 12.2 volt, after 1/2 day resting, seems kinda low. Our original battery will get down around 12.45, after 2-3 days inactivity. It was installed new in nov 2010, orig battery was doa.

 

Maybe there is too much air intake of your ICE (you hear a different sound). A small hole in your supply pipe to the engine ? An animal bite or something like that? Brittle pipes?

 

4 year old 12 v and problems, I'd also check the emergency brake, just a slight tightness would be a lot of friction, freeze thaw cycles move water to weird places.
Silly as it seems a clogged interior air filter causes wasted energy.

 

12 volt should be at least 12.5, a new one is 13.0+ might as well replace that first and see where you stand.

 

IMO swapping out the 12v battery is at best going to make a barely perceptible difference in fuel economy. It isn't going to do anything for the rough ICE. Swapping out the 12v at your convenience is much better than doing it when it dies which is mostly likely going to be in a less convenient place at a less convenient time.

Are the tires at their usual pressure?

My list of suspects would include all the standard ICE suspects.

 

My gosh 65 mpg lifetime, and 50ish in this PA colder weather is amazing.

Do you have a pip or a reg prius, which trim level?

I'm in PA as well, and granted I have a 2015 Persona with the stock 17"er handicap, my lifetime for the last few months is right around 40-40.5 mpg calculated, 43.7 displayed on trip b as of today with 3200 miles on the odo. I guess the fact that I've only really owned the car during this cold weather doesn't help my lifetime average. I don't drive fast at all and have my tires set to 44/42...and have my lower grill blocked also.

And FYI my commute is pretty much the same distance as yours.

 

For another potential issue, see also: Front brakes seized?
I've have similar problems on a couple previous cars, front and rear, not my Prius.

 

Allowable range for Odyssey:

• 11.7V
• 12.84V

Source: http://www.odysseybattery.com/documents/US-ODY-OM-012_1014.pdf

Bob Wilson

 

not nit picky at all. it sounds like you have an issue, and it's definitely affecting your mpg's. but i would not ignore an engine shudder. maybe there is a code stored.

 

what is this bob? do they make one for the prius? never mind, just checked the website, nope.

 

The battery doesn't care.

Bob Wilson