Engine Performance problem
1999 Kia Clarus 4 cyl Front Wheel Drive Manual 242000 miles

Hello. I have a Kia Clarus 1999 2.0 GLX

When I am driving economically and not coming over 2500 rpm the consumption is ok, but when I am on a Danish highway where you are allowed to drive 130 km/h, I require to drive with 4000 rpm in 5'th gear.
On highway the consumption was about 4 liter for 18 km which sounds insane when the car should drive 9,7 km pr liter.

I want to ask, wether it might be a problem with turbo or fuel injection and that kind of stuff? If so, who should I go to, to solve the problem?

Thanks in advance,

Alex