"fair to speculate" is correct and usable in written English.
You can use "fair to speculate" when you are making an educated guess or forming a hypothesis, and you need to qualify it as an opinion. For example: "Given the current market conditions, it is fair to speculate that the stock will go up in value."
So it is fair to speculate.
But he said it was fair to speculate about whether the company can survive over the longer term.
"I think it's fair to speculate that there was something very small, not detectable on the M.R.I.," Cashman said.
At age 36, it's fair to speculate that this season is the beginning of a downward spiral.
While the terms of the settlement are supposed to remain confidential, it's fair to speculate that the players received a lucrative amount to end their lawsuits.
It's fair to speculate that the idea of Nadal eclipsing his Slam number eats at him, as does losing to him again and again.
As an acquirer, Snapchat has a history of scooping up smaller companies that it thinks will make great features for the app; it's a fair assumption to speculate that recent hits like Prisma could be next.
Burt here's Tursk: "With World Cup about to reach it's culmination, I think it's quite fair to start speculating on who will get the best player of the year / Balon d'Or for year 2010.
Indeed, it may not be inappropriate to speculate that some fair portion of the respect generally shown to Whitehead by his philosophical peers at the time arose from their sheer bafflement.
One might have imagined, if asked to speculate, say, yesterday, a fair number of reason for Petraeus to resign at some point.
Fair enough, maybe -- except that she uses the scene to speculate on Mrs. Clinton's motives for running, and without evidence or sources introduces the idea that Mrs. Clinton wants to be senator to set up a run for president.
