First, check your purge control valve, if it sticks open it'll kill your fuel economy. I clean mine every 50,000km. The valve has a design flaw in the 04/05 model car. I never bothered to let them replace it at the dealership, so if yours wasn't replaced, this may be why you're not seeing good fuel economy.
Next, I neglected to mention I drive a manual transmission, I'm sure it makes a few MPG difference but based on your reaction, I'll assume your results are WAY off mine.
If I drive like I have a deathwish and pretend the gas and brake pedals are digital inputs, I can still get 600km out of a 52 Litre tank. If I simply accelerate normally from stoplights, and pay attention to keeping the throttle constant when cruising at highway speeds, I'll easily hit 800km a tank. I'm not talking about hypermiling, drafting trucks, and coasting downhill with the engine off... I've heard people pass 1000km on a tank doing crazy shit like that, but that isn't real world.
I'd highly recommend getting a scangage or similar device and hooking it up to your car to see your instantaneous fuel consumption. Very small changes in how you handle the accelerator pedal make a world of difference on your fuel consumption, and without it affecting your ability to keep up with normal traffic, hell I average 20km/h over the posted speed limit on highways, and I really do get over 40MPG.