Chevrolet: my fuel economy may not be that bad

In the last week, I drove on the highway, and, ....

My fuel range kept getting higher, even though I drove well over 200km.

Recently, I filled my tank, it read around 400km range. Again, drive over
100km, and it is reading almost 600km.

My fule economy has dropped down to under 10L/100km.

I guess that sensor is not that accurate...

Does anyone know what the heads are made of on the 04 3.5L V6? I am at over
140000km, and will be doing plugs soon and wondering how painfull that will
be.

Thanks,

Micheal