Yes it is possible. When you send sms to arduino, store the new value to EEPROM and assign it to OverSpeed variable seperately. And add a line to setup function of arduino that reads the eeprom and update the OverSpeed variable.
Reason for updating in setup is, you specify the variable when you are burning the program. If you dont use eeprom, you will be fine untill a reset. Reset will wipe the last changed value to initial burned value. But if you bypass it in setup function, its just what you want.