LOW MPG (I think), plz help

I drive 2005 Prius with 208,000 miles on it.

- Have you read This Thread Yet?
YES -- but the link in this thread is dead

- What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)
43-46 MPG from prius trip computer

- What fuel economy are you expecting and why?
50+ since other 2G prius in my area got about that number

- What are the approximate outside air temps?
about 80F

- How long are your trips?
Got about same mpg for 2miles, 80 miles, or 600 miles trip (never reach 50)

- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?
About same mpg for both city and highway. Avg mph is about 20-30mph city and 65-75mph highway

- What region/state are you in? (if you haven't set your location in your profile)
Columbia, MO

- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)
I would say gentle hills

- Is your oil overfilled? (i.e. above the full mark on the dipstick)
No

- How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night?
New one about 3 years ago but I did check it still got 12V after sit overnight and 14V when start the car

- Have you had your alignment checked? Any pulling or abnormal tire wear?
New tire about year ago.

- Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15).
No, I got Bridgestone Ecopia 185/65R15 86H on both rear tire and front right tire. On the front left tire I got Yokohama Avid Touring S 185/65R15 86S.

- What are your tire pressures?
40 front tires and 39 back tires

- Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.)
I used to drive 2002 Dodge stratus coupe, V6 engine autotrans. Got about 25-29MPG highway but much lower in city may be about 8-12MPG

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?
I drive very careful, start slow and smooth braking. I even use pulse&glide whenever it applicable.

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?
No.

- Are you driving using D or B mode?
D

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to?
It normally turn off the fan and AC. I rarely turn on the AC.

- If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute?
I wouldn't say it a drop on mileage except that when I was test driving it, I got about 58MPG. But after I bought it, I never get near 50MPG at all. And got yellow engine check with code P0420. I haven't replace my cat converter yet. However, since then I have replace the oil(mobile1 synthetic), best bosch oil filter, bosch IR fusion spark plug, transmission oil with OEM. The MPG stay roughly the same before and after I did all the replacement I mentioned.

Another thing, when I bought the car the oil was overfilled. So I drain it out until the oil drop half way between low and full in drip stick. And I did perform throttle cleaning (remove the air filter box and spray throttle cleaner to throttle plate and brush out the stain). But it been an oil change since than. I do my own oil change and the oil level is still the same (halfway between low and full). Also, I try to use chevron techron fuel system cleaner on every other tank of gas (I have use cataclean too it a wast of money).

 

First off let me say thank you to everyone for sparing their time to answer my question.
I did spend quite sometime reading about P0420 code . Here is my theory on why I concluded on bad cat which might be associated with reason that I got 58mpg while test drive. You see right before the dealer sold the car to me, he got oil change which the stupid toyota dealer overfilled the oil (almost an inch above full level on dipstick). And I believe that the oil filled oil leak into engine and got into cat. So while I was test driving (I know it kind of stupid but I only test driving a few exit on highway) the car still work fine. But after the few week of personal used the cat gone bad and I got the yellow light, the car no longer working properly and so my MPG drop. Beside, this cat already got 208,000 miles on it. Please let me know if my theory is even possible.
About checking battery, I do the signal monitor in inspection mode through MFD. Is there other way to check it for sure if battery is at fault here?
My plan the next replacement will be a new set LLR tire too (wait for the next paycheck). Do you guy have any suggestion on tire brand/model? Especially the one that Walmart carried would be nice.

 

Update on the tires:
Actually, it's not 3 different brands of tire but 2 brand: 3 of the tires are Bridgestone Ecopia (both rear and front right tire) and the odd one is Yokohama Avid Touring S (front left tire).
The problem is both front tires (Ecopia and Touring S) still have nice a thick thread (if I remember correctly both front tire only have 20k miles in them). So the question in my mind is necessary to replace all four tires? Or I should replace only rear tires and rotate it to front?
Please let me know what you guy think.

 

Well, I haven't check the alignment yet and the both rear tires's thread are really thin. I think it time to change and the problem is I'm not sure whether to change out all four or just the two that are bad (both front tire's thread still pretty deep).

 

Why? I thought that the new pair of tires should be put in the front and rotate the current front pair to rear wheel.