Unethical if you submit for reimbursement for purposes other than which the company intended. Ethical if they don't care how you traverse the miles. The only reasons not to ask would be if you expected to be denied, or if you thought they'd think poorly of you for suggesting it. Therefore if there is any question in your mind, and you submitted for reimbursement just the same, it would be a failure of ethics. So just ask, or forget about it. If you want to be ethical.