Why is my car revving high when I get past 60km/hour?

The vehicle listed above is a nt30 with just over 200,000 km on the clock. It starts okay, idles okay and kicks back ok during gear shift. Revs remains okay (1,500-2,000 RPMs) when I drive from 0-60km/hour or maintain that speed. Problem starts when I go past 60km/hour speed, the rev goes up with speed! To get to 80km/hour even on downhill, the rev has to be 3,500 RPMs. To get 100km/hour, the rev is 4,000 RPMs and up it goes with higher speed. I maintain a gentle accelerator pressure for car to pick speed for itself and the engine does not whine during these high rev speeds, I even checked with a diagnose-r if all the gears shifts and they do. I Changed transmission oil, changed transmission solenoids, changed spark plugs, checked the speed sensor. And all these have yielded no results. I checked the fuel pump and checked the fuel filter, and it is okay. I even checked just to be sure that my clock is working okay. Did a full tank and drove on a highway at the high revs and the fuel was guzzled up fast? I am lost for what to check and I need help. The high revs are not economy friendly and driving at low speed is neither an option.