speculate about someone or something
to make guesses about someone or something; to hypothesize about someone or something. I refuse to speculate about Sally. I don't presume to guess what she will do. We don't speculate about the future.
speculate in something
to make risky business deals in the buying and selling of something. Jeff made a fortune speculating in cotton. I do not wish to speculate in anything. It is too risky.
speculate on something
to make a hypothesis about something. I really don't want to speculate on what might happen next. Would you care to speculate on what might happen if you quit your job?