Huge Drop In MPG

My 2008er with about 75k miles had a huge drop in MPG last week. Its cold here in northern NY and I usually drop from around 50 MPG to mid 40's in the winter. I was getting mid-high 40's until last week, when I noticed the car felt like it was working harder, now my display says @32 mpg average and manually I got 29 MPGs since last fill up.

I drive 55 mph rural highways mostly with lots of moderate hills. When driving down hills, I can't coast 55 mph and need to accelerate downhill to keep speed. When driving down a hill, if I shift to N, the car feels like something lets go and the car really speeds up, so I don't think its a tire inflation, brake or alignment issue. If I shift to D, it slows down like I put breaks on or feels as if I am shifting into B. If I really shift into B, it slows down more than D, so I would not think there is a problem with the regen brake mechanism being stuck or something.

It seems like it always wants to charge the battery. Even when the car is working hard, like driving up big steep hills, the display shows the gas engine sending energy to the electric motor and onto the drive battery.

The oil is slightly below the high mark, so I don't think its a case of too much oil.

About a month ago, I returned from a 5 day trip and the car would not start. I jumped it and has been starting fine ever since, even after sitting unused for another 5 day stint. I checked the battery using the screen display check and one morning it started off, without fully starting the car, at 11.7V. After turning the headlights on, it started dropping and it got 10.8ish before I started the car, after which it jumped to 14V.
About 18 months ago, my wife ran out of gas. The drive battery died. We put gas in it and still would not start. After reading about it, I assumed that we killed the battery but we tried jumping it it started and it worked fine since. Even giving me average 56 MPGs this past summer.

I live over an hour from the nearest Toyota dealer and just don't have time to take it in anytime soon. Any help is appreciated. Thanks.

 

I am ordering the Optima yellow-top right now, but I just checked the display check for the battery again and it was at 12V. Should it be higher on a good battery even after sitting overnight at -10 degrees?

 

Thanks a bunch for all the responses. Prius Chat is awesome!

Amazon is about $100 less than local parts stop, but unfortunately sold out of the yellow top right now. Any recommendations for online places to get a battery at a good price?

 

Nevermind, I just clicked Jimbo's link above. Thanks Jimbo.

 

Update:

I have my Optima battery but its been too cold to install, then it warmed up but rained, now its cold and snowing again. But...my wife has been driving the car because she works close to home. Every morning before I started the car for her, I checked the battery. It was 11.7, 11.8. The weather warmed up and I checked it yesterday morning, 12.2V. Last night it was 12.3V. These are all readings after hitting the button once. So, last night I took a ride to see how it was driving.

At first, I was at the end of the block and slowing for a stop sign. I had my foot on the brake and the current MPG was dropping from teens, then 9.2, then 4.something, and then 0. I thought when I usually am slowing down, foot on brake (not on accelerator), the MPG should be 99. Don't know if that's normal.

I too a 20 min drive on highway and local roads and the car ran normally. 48.3 MPG average. I am going to replace the battery because its time, it was dead once and because I already bought it. I wonder if the weak battery and the cold weather was causing the problem.