BAD back to very GOOD FUEL Mileage

I bought my 2008 prius used back in September of 2011. I Usually got about 41-44 mpg since when I had gotten it. It had 39,9XX on the odometer when purchased. I drove mainly highway miles from Ocala to Jacksonville and then the car sat apx 2 weeks and then made the trip back from Jacksonville to Ocala, and then 2 days later it would make the return trip to Jacksonville. At the time I was an over the road truck driver, which is why the car sat so much. One day, I got back off the road and needless to say the car wouldn't start. I'm not new to cars....but it took me a moment to find where to jump the car using the owners manual to figure it out. Anyways the car jump started and all was good.

About 4 months ago my mileage dropped to 42, then 41, then to about 39.75 mpg and to me the car seemed to be running more (as in idling at stop lights when it was normally off. I also noticed (or thought I noticed or vaguely remembered):

1. All the buttons on the steering wheel looked more dim than usual,
2. with the car off the dome light was very dim
3. the smart key system was problematic (even though I replaced the batteries twice)
4. If I opened the car door with the car parked and running, the interior light would be somewhat bright, but if I turned the car off and left the door open the light would go strangely dim quickly
5. Sometimes the car would not want to start (thought it was the key fob as everytime I put it closer to the ignition or inserted it into the ignition, the car would start.
6. The Multi function display would show some error message about parking on flat ground and shifting to PARK (when the car was being started, was already on FLAT ground and in PARK.

I also thought maybe the oil has degraded as I run mobil 5w-30 synthetic extended oil rated for 15000 miles and a fram ultra which is also listed as good for 15000 miles. At the time the car had 8000 miles on the oil and filter.

I also thought maybe because it was hot 90+ outside that maybe the auto ac was helping chew the fuel up.

So when in doubt, I googled it and this forum came up. It pointed me in the direction of me needing possibly a new 12 volt battery. I checked the voltage with the onboard diagnostic tool via the MFD (can't remember whose post I found it from), but it was a real life saver. Found out that my battery was only holding 10.1 with no load and only registering 9.7 with a draw. So I decided to go with (again suggested from this site) optima yellow top battery (which is the one manufactured specific for this car). I chose the optima yellow top for 2 reasons, first optima battery is well known for good batteries and second it was only 20 bucks more than a lead acid. I'm mechanically inclined and the whole job of installing the battery took 1.5 hours (1 hour to unload the random junk that I had in the trunk and a 1/2 hour to actually put the battery in lol).

Amazingly my MPG shot back up to 44.1 mpg on that same tank. I was pretty happy . It was getting that on the same oil that was in the car too. So it was definitely the battery.

I decided it I had put on 15000 miles on the car since I bought it, and a new air filter was due. I kind of like K&N filters. I've ran them in several Mustang GT's, a full size Dodge quadcab, truck, and on my hayabusa motorcycle and everytime it gave a little more feel as I gained some horsepower and saw about a 1%-2% mpg increase when used. I found a new one on ebay new for 39.99 with free shipping and said "I have to try it". Orderd it and recived it, put it on my car and on that tank of fuel that it was installed on I saw a average mpg of 47.25 mpg (it was installed at about half a tank and when the car was filled the MFD said 46.1 and calculating it miles divided by gallons it came out to 47.25 mpg. I'm on my 1st official tank with the K&N and things are looking promising with the MFD indicating 50.2 mpg at the half tank mark and 270 miles on the trip odometer.

Anyways, thanks for the people that posted their problems and suggestions which ultimately helped me out. The Site is great

It's my first posting, So I'll share a little about me:

I used to be an over the road Truck Driver and got tired of being gone all the time...so now I am a truck driver in hiding...lol. I now work on semi trucks...ironic twist of fate, but I love being home everyday.

When I purchased my prius I traded in my dodge truck...and the dealership I was at thought I was joking around. I was paying over 500 a month in fuel for the truck at 14.1 to a high of 15.1 mpg and It was costing 90+ to fill it. I walked out of the Toyota dealer and my car payment, insurance and gas are still a couple hundred dollars cheaper than what I was paying in gas. I'd say it was a win win situation

I enjoy riding motorcycles I have 3
a 2005 Suzuki Hayabusa (GSXR1300) that I was hit on March 16th of this year. I made out ok, and after arguing with the insurance company, they agreed it was in their best interest to pay for the damages and fix the bike versus trying to total it for cosmetic only damages. This bike gets worse fuel mileage than the prius, but it is fun to ride.
I also have a 1998 Yamaha Thundercat this bike gets about 45-50 mpg
I have a 2013 Kawasaki Ninja 300 special Edition and it gives 56-57 in town and 64-65 on the highway, but the prius is more comfortable than the ninja.
I also have a 1990 mustang gt that probably should get driven more often.