I shift In between 2-3K rpm, 2.5 to be exact, there is no rough driving what so ever, I have had cars before, the mileage is unbearable, the other cars I told have those statistics that are legit, everyone knows about it.
I just need advice on spark plugs/ air filter or something.
any great tip to boost up the mileage,

400 KM is very low for a 1.6L car that is being driven smoothly.