2007 Kia Rondo - Terrible fuel economy

Recently bought a used 2007 Rondo EX V6, and one of the first things I noticed is that we are getting terrible fuel economy. A full tank in the vehicle only gets us ~300 - 350 km, and the on board L/100km monitor reports 15 to 17 L/100km. Driving conditions are mixed city and highway, and we do have a leak in the flex pipe but I dont think the effect should be that dramatic.

Any first steps I should take to try and solve the issue? We love the car but avoid driving it due to this expense. Any help is appreciated.