Any constitutional charter city in this state which now has or may hereafter acquire a population in excess of six hundred fifty thousand inhabitants is authorized to levy for local purposes on the class of property enumerated in section 92.041 hereof an ad valorem rate of taxation less than that levied by such city on real estate or other property for the same purposes, and said reductions may from time to time be arranged to apply on both or either the tax rate for payments of valid indebtedness or the tax rate for city purposes.