Which affects fuel economy so much?

Recently bought 2010 Prius (I love Prius cars, this is my third Prius) and currently having really bad mileage despite of changing oil and filter (used premium stuff, full synthetic 5W-40 oil etc.) and using only 98 Octane fuel, I still get only 600 km out of 35 l tank (5.8 L/100 km here in Australia, 40 mpg in US). I used to have Gen 2 Prius which I loved so much and it gave me 4.6 L/100 km (52 mpg) and this Gen 3 model is supposed to be better...! I know how to drive a Prius (which is why I got excellent mileage records for my old model) but this thing doesn't return me better results.. Engine sound is normal and everything else looks and feels normal about the car except for the economy. I need your guys advise on possible courses..

1) Fuel pump issue?
2) Injector blockage? (I used injector cleaners couple of times already, doesn't seem to help)
3) Air filter? (I have yet to check this)
4) Automatic fluid? (Car has only done 70,000 km, do we need to change Automatic fluid? I changed my old Prius oil only after 230,000 km and it ran like a dream even without changing oil)
5) Auxiliary battery? (Checked Voltage and it is around 12V)
6) Traction battery? (No sudden discharging of this, no irregular/sudden charging cycles, so I presume it's performing alright)
7) Spark plugs? (I have yet to check)

I don't know what else could it be.. Even if it's one of above things, how come it would be that bad? (Unless it's combination of many issues to which I'm afraid..!)
Waiting for your valuable feedback guys...
Note: It's the Winter I should tell you, also the tyres are not particularly fuel saving/low resistance ones
(Unfortunate thing is, I always remember my good old 2nd Gen Prius, which obviously performed much better even in Winter..)

 

Thanks for your reply mate, I honestly don't think 5W40 would be an issue in Australian climate.. In fact I used to have the exact same oil in my previous Prius and got fantastic results. I trust this particular oil so much to a point that it can not possibly be the factor..
Battery voltage,
after 3 days idle it came 11.8
5 mins after start it came 12.1
after driving it came 12.2
I'm going to do a ATF change as you advised and see if that helps..

 

Thanks for your reply mate..
She's got 73,000 km (about 45,000 miles)
No maintenance record at all so I'm not sure about PCV valve change.. However I do know, the odometer is accurate as I did a history check on the car.

 

Thanks for your reply mate..
Yes, I'm going to do ATF change ASAP..
About wheel alignment, actually it's pretty bad. You got all facts right, this car had been in an accident and repaired later. As it can be expected, the wheel alignment was odd and I noticed this during my first drive.. I should do this as priority..
Also the rear wheels are two different type of tyres, I believe that could be affecting fuel economy as well..
About the warm up things etc.. I have had Prius cars for quite sometimes now and I think I know how to drive them and how to achieve high mileage.. But this particular car doesn't seem to respond well enough..

 

As I mentioned earlier, I don't think this drop in economy has got anything to do with 5w-40 oil which I have always used in my previous Prius cars (and have had excellent performance both pick up and economy wise).
You've got a good point for AC though.. AC is on full time however AC was not a huge factor with my previous Prius.. (2nd Gen)
Ex: with AC 4.8L/100 km, without AC 4.4L/100 km
Anyways, I'll try driving without AC just for the sake of checking this..

 

Thanks a lot for your valuable answer.. While I do believe it could be one of the factors, I still can't believe it's causing 10 mpg drop which is simply not factual. Let me explain why I think so.
12V battery gets charged within first 10 km or so of driving so first 5 or 10 minutes of driving a drastic drop in mpg is totally understandable due to extra effort from traction battery/engine to charge the 12v battery to it's full capacity (and getting car's AC to the optimum temperature by running engine more as it's the winter).
But after 12V gets it's optimum voltage (say 12.8 which I'm pretty sure it's effortlessly maintains until the end of the trip as drop in voltage is not significant during driving) and car's AC is maintained in fairly insulated interior, one can expect a better mileage afterwards.. Right?
But when I turned on the 5 min consumption graph on the screen, it's still showing above 5.5 L/100 km consistent fuel consumption throughout the journey even if I drove 100 km.. (With my previous Prius, the first two bars would go above 5 and then usually drops to 4.0 range when cruising at around 70-80 km/h optimum speed for Prius) But this car doesn't do that which is pretty strange.. Which is why I think it's neither AC nor 12V battery which causes the issue.. It could well be ATF or some other serious issue that I can't simply figure out.. Any ideas guys?

 

Thanks for the reply mate... I also think it's a combination of factors.. By the way, I have two Prius Cars, Prius C and Prius 3rd Gen and this query is for the 3rd gen Prius - I don't know how to add both cars under my profile.. LOL (I used to have Prius 2nd gen to which I always used 5w-40 oils and had excellent fuel economy).
I'll change the air filter first and drive for a week to see if that does any difference, then I'll change the battery.. I'll update you guys accordingly..