Poor Fuel Economy

[Justcaz]

Hello..
Please forgive me as I know little to nothing about cars...
Especially my poor Yaris.

Basically I've got a 2008 YRX 1.5 litre Yaris.
It's had no obvious or major problems.
I mostly use it for suburban driving... it does do short trips, 5-10 minutes daily. But also longer trips on occasion.

I have noticed my fuel economy decreasing a lot over the last year.
To the point now where I am getting about 480km out of a 42 Litre full tank.
Thats over 11 litres per 100 kms.
I don't drive like a complete rev-head.. I do get stuck in traffic quite a bit.. but I can't really think this would be the reason.


Too me something is just not adding up.
Can anyone think of any obvious problems that might be causing this.. it would really be a huge help to me.

Thanks