speculate about (someone or something)
To conjecture, hypothesize, or make guesses about someone or something without having supporting or conclusive evidence. We've all been speculating about the mysterious man who moved into the house two doors down. It's certain to be a gamble, but until we try it, all we can do is speculate about how the product will do on the market.
speculate in (something)
To buy or sell some asset, such as commodities, securities, property, etc., at a risk while hoping to gain a profit from short-term market fluctuations. My uncle made, then promptly lost, a fortune speculating in oil wells in the 1980s. You'd be a fool to speculate in dairy in today's market.
speculate on (something)
To conjecture, hypothesize, or make guesses about something without having supporting or conclusive evidence. You can't just speculate on what effect this legislation will have on the country—you've got to have some concrete data to show it will work! Instead of spending your energy speculating on the future, you should focus on the present and do what makes you happy.
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.