High Rev & Fuel economy advice

[pHysiX]

Hey there everyone. I just picked up my Jazz (2nd hand car, done 140000km) a few nights ago n i'm totally happy with it so far. Only issue:

Driving on a freeway (80-100km/h), i noticed that my engine was revving @ 3000rpm. Is this normal? I'm on a manual, so naturally i put it into 5th gear.

So far, my fuel economy is a bit unstable. I started out doing 4.0L/100km, but currently am sitting on 5.1L/100km according to the digital meter on my Jazz.

Im not accelerating hard etc, i've made sure my tyres are @ optimal psi, and i'm changing the air filter soon. does anyone else have any other ideas? asides honda feo, what other oil & rating should i use? 10w30 for my car would be alright? i'd love to keep this car as fuel efficient as i can

btw, what's the fuel tank capacity on a 2003 Honda Jazz GD Gli Manual?

[Gnat]

Well, if its in 5th, thats as low as the revs will go. They could do with a longer 5th gear, but there is so little torque down low it probably wouldn't worth it anyway.
5.1L/100ks is pretty good, I averaged ~6-6.5L on an urban cycle, down to 4L on the freeway runs.

As for oil, I used Mobil 0w20 Synthetic, but the engine had done a lot less Ks.