Ethics is two things. First, ethics refers to well-founded standards of right and wrong that prescribe what humans ought to do, usually in terms of rights, obligations, benefits to society, fairness, or specific virtues.
If you mean the average speed limit, it's probably 65 MPH. In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits. About 30 states have limits of 70 MPH or higher. A few Western states have no speed limits on certain stretches of highway.
As an exception, state law defines the speed limit to be 65 MPH on certain highways in Delaware and Massachusetts. States where the DOT has not used its authority to increase speed limits are Alabama, Indiana, Minnesota, North Dakota, Oklahoma, and South Carolina.