Demio mysterious bad mileage

[Stasi]

ok, gurus, can you help me with this one.
I have a Mazda Demio 2 (2004) 1.3 Its a pretty nice car, roomy, powerful, reliable, smooth, runs great.
Only problem is that it gets horrible mileage. I can barely get anywere close to 400klm on a full tank fill-up. It has i believe a 45 ltr tank plus reserve. But no matter how carefully its been driven, even tried all highway, can't get more than 400klm. usually 350-375. Isn't that really bad? What the heck is going on?
I thought it might be a computer fault because it runs perfectly, there is no indication of anything being the matter. Passes inspections without a hitch, emissions etc are perfect.
So, what gives folks?

[Draigflag]

This is one of the older Mazda, with high emissions and poor economy. It's rated at between 32 and 47 UK MPG here, which as we all know, are optimistic anyway, so despite the cars small size, I would expect pretty low economy unfortunately. The tank is 43 litres, assuming the warning light comes on with about 5 litres left, then you're looking at roughly 500 km per tank minimum. So yea, I guess it's a little lower than average. What kind of trips do you do? Top speeds do you travel at etc?