The New York Daily News and the last of its 11 unions reached tentative agreement at 6 a.m. today after all-night negotiations on cost-cutting measures that will save the ailing paper an estimated $50 million.

The agreement reached with leaders of the drivers' and pressmen's unions must be ratified by the membership on Sept. 3. The other nine unions reached agreements with the News earlier.

The savings, which the News management said were necessary for the tabloid to survive, would come from buy-outs, elimination of jobs, and changes in work rules. The 11 unions would agree to cut the equivalent of 1,340 full-time jobs.

Still unresolved is the issue of job guarantees for those who remain.

During the negotiations, union leaders said they needed assurances that the News' owners, the Tribune Co., would continue to publish the paper for a period of time. As a result, the News offered to guarantee jobs for three years beyond the conclusion of the current contract in March 1984. The News also agreed to renegotiate wages if the newspaper earned an annual profit above 6 percent of sales.

The unions will discuss job guarantees at meetings next week.

It is unclear whether these guarantees would have to apply to New York's other two major papers, the Times and the Post. Under the terms of the papers' joint bargaining agreements--called "me-too" agreements--concessions granted by unions to one paper apply to all three.

But a News spokesman said "there's not a lot that's me-tooable. Most of these agreements apply only to the News' unions."

The News spokesman called the agreement "extraordinary." He said that, by getting concessions worth $50 million, the News had "achieved its target."