He did not speculate on the reason for the difference.

I didn't want to speculate on the nature of his business meeting.

I didn't have enough information to speculate on that yet.

There was no more time to speculate on the subject, for the present.

We can speculate on that all day without getting an answer.

Hard to speculate on that with the information at hand.

None of them could speculate on that, so they moved forward.

But I knew better than to speculate on his or her identity.

Speculate on the way if you want, but keep cool now.

But he would not wait to speculate on the agent!