Airspeed

Hello guys.
i Would like to know what is the relation between KIAS/MACH and KM/H..i know for example that 1 nautic mile or knot is 1.8Km...but i have no idea about the ones mentioned before.
Also, why does the speed limit comes down as long as u go higher with the airplane?
E.g. FL11 340Kias...FL34 290 Kias.
thank you buddies
see u
i Would like to know what is the relation between KIAS/MACH and KM/H..i know for example that 1 nautic mile or knot is 1.8Km...but i have no idea about the ones mentioned before.
Also, why does the speed limit comes down as long as u go higher with the airplane?
E.g. FL11 340Kias...FL34 290 Kias.
thank you buddies

see u