My altis 1.6 cruisetronic is dropping its rpm and not going above 170kmph

[anasmahmood319]

My altis 1.6 cruisetronic is dropping its RPM and not going above 160kmph and makes the missing above 140kmph
Please guide me
Im very tensed
While i changed the plugs original recently

[pwuser155202808428]

170 bhot hein is ziada speed per merna chahtay ho??