I have recently bought a Toyota Corolla Altis from a cousin of mine. The car is generally in a very good condition but it seems to have some issue with it's RPM. When I hit 100kmph, the rev is almost on 3 (3000 rpm) and goes up to 4 (4000 rpm) on hitting 120kmph only. This causes A LOT of engine noise and obviously affects fuel consumption too. The engine has a lot of power and keeps on accelerating smoothly to 160+ kmph but the engine noise makes it almost impossible to drive at high speeds.
The car was converted to CNG but driven only around 5000km before taking the kit off.
I have consulted 2-3 different mechanics and all seem to be clueless about the issue. One of them even said "yeh is model ka masla hai".
I have also attached a couple of pictures. Any help would be greatly appreciated.
Toyota Corolla Altis 1.8
About 75,000 km done