Vitz wont go beyond 140-160 mark. Dont keep hopes high, And dont fiddle with the car aswell.
Just get a routine maintainence done on your car, If you dont know mechanics get it done from a Toyota Dealer ship, At maximum, Your cars cylincer Catalyst could be choked.
Other than that the CVT Vitz is not a fast car. I have traveled from Lahore and to Lahore on motorways it manages 160+ only on slopes. (im talking about the 3rd Gen) 2nd Gens are slower then the 3rd Gens.
120-140 is good average,