What is motor kv?
KV is rpm per volt.
If you run 45KV motor at 48V than the no load speed will be 2160 RPM
(45(rpm/volt x48 volt = 2160 RPM)
If you will run the same 45KV motor at 72V than the no load speed would be:
45x72= 3240 RPM
It’s important to say that the typical max motor speed under load is about 90% of the no load speed (the max speed depend much on the load).