It’s the best-kept secret in all of New York: How exactly does Gov. Cuomo plan to pay for the $3.9 billion new Tappan Zee Bridge that even now is being built?

Progress on the span, a centerpiece of Cuomo’s 2014 re-election campaign, is impressively rapid — but Team Cuomo still hasn’t dropped a public plan for where the money’s coming from.

A top suspect is the New York Thruway Authority. But it just adopted its 2017 budget with yet another promise not to hike tolls on the state’s premier highway until at least 2020.

The authority says it expects to be flush, as a dramatic rise in travelers on the 570-mile road brings in an extra $11 million in revenue. But that wouldn’t begin to cover the financing for Tappan Zee construction.

And not all the state’s revenue projections pan out: Just last week, Comptroller Tom DiNapoli reported that tax revenues for the first half of the current fiscal year fell short of the governor’s projection by nearly $1 billion.

But the Tappan Zee remains the biggest mystery in the state’s finances. Notably, the governor still refuses to make public the financing documents his administration had submitted to the feds last year, in a failed effort to score a $511 million environmental loan from Washington to use on the new bridge.

E.J. McMahon of the Empire Center for Public Policy has suggested Cuomo “is trying to put off paying the costs of a new bridge until he leaves office.”

With no sort of public plan or explanation in view, that sounds about right.