The Federal Trade Commission yesterday said it had given final approval to a rule that requires used-car dealers to put window stickers on cars giving warranty and other information, but eliminates an earlier requirement that information on the known mechanical defects in a car also be included on its sticker.

The rule, approved by a 3-to-1 vote on Tuesday after preliminary approval had been given in July, has been roundly criticized by consumer advocacy groups as being too weak to prevent many used-car dealer ripoffs of consumers.

Commission officials have argued that a study done in Wisconsin of that state's defect-disclosure laws showed that consumer benefits from the disclosures were minimal.

The original measure, approved by the commission in 1981, was vetoed by Congress the next year. The Supreme Court, however, ruled that veto unconstitutional in 1983, forcing the commission to pass a new rule.

Under the new regulation, which is to go into effect next May, dealers must put a sticker on the window of each used car listing warranty terms; an indication of whether the car is being sold "as is" so that consumers would have to pay for any repairs; a suggestion that the consumers seek an independent inspection of the car; a list of major defects that can occur in used cars, information about availability of service contracts, and a warning that consumers get all dealer promises in writing.