yes, its over drive button.
If we put it off, it doesn't put in 5th gear, this is according to my knowledge, you might differ from it.
Nope, no midifications. Its default, just 1 modification is done which is addition of cng in my car. I didn't want to do that but watching the petrol prices raising higher made me do this.
Yea, once i tried to do it, i reached 110 with o/d off. Actually, as you know, when we put it off, it feels like its putting pressure on engine, thats why i put it on before reaching 100.