If everthing seems normal what could be causing a sudden increase in fuel consumption on a 1999 Isuzu Trooper 3.2 petrol. The average consumption has gone from 7.5 kml to 6.5 kml?

Check for a leak somewhere in the system. Tank - lines - etc. Park on a dry surface in a parking lot. Turn the engine off, then turn the key "ON" without starting the engine. Give it a few moments then look to se if you can see petrol under the vehicle.