speculate on something
speculate about something

One can only speculate about the use made of the park.

But he doesn't want to speculate about the next project.

But for the most part we can only speculate about these things.

Instead of the future, these stories speculate about the past.

But she would not speculate about plans for the lot.

He would not speculate about any position that might interest him.

But Sam had made up his mind not to speculate any further about the girls.

I can only speculate about the reasons for their success.

He added it was too soon to speculate about management changes.

They never speculated about other species and don't know what to make of them.