NBC and ABC were tied through three weeks of the May sweeps, with an 11.7 rating each, before “Cheers” turned the lights out on any suspense in the monthlong ratings survey.After Thursday night, NBC enjoyed a near-unassailable lead in terms of claiming its ninth consecutive May Nielsen crown, improving its average by 1.1 rating points to a 12.8 rating, 22 share, up 6% compared to the same period last year. By contrast, the other three broadcast webs were down through Thursday, with ABC (11.5/20) off 3%, CBS (11.0/19) sliding 6% and Fox Broadcasting Co. (7.4/13) down 9% compared to year-ago figures. NBC carried Thursday’s momentum into the weekend, tying for first in Nielsen overnights both Friday and Saturday, though CBS should rally to take the latter night in the sticks. CBS will nevertheless have to rally sharply with itsminiseries “Love, Honor and Obey” to have a chance at catching ABC in the race for second place. The 98-minute “Cheers” finale (45.5/64) drew the fourth highest rating in history for any series program (behind only the final “MASH,” the answer to “Who Shot J.R.?” on “Dallas” and the apprehension of the one-armed man during the final “The Fugitive”). By way of comparison, last year’s “The Cosby Show” finale drew a 28.0/45 and February’s Michael Jackson-Oprah Winfrey special a 39.3/56. Attracting more than 93 million total viewers, the “Cheers” rating surpassed this year’s Super Bowl to rank as the top-rated program of any type since the 1987 edition of the game. The episode becomes the 20th-rated program of all time , joining a list that includes nine Super Bowl telecasts, three installments of “Roots,” NBC’s initial two-part 1976 broadcast airing of “Gone With the Wind,” and ABC telefilm “The Day After” (46.0/62), the last entertainment program to achieve such a rating, which aired Nov. 20, 1983. In terms of demographics, “Cheers” served up a staggering 76 share among adults 18-49, faring worst among viewers over 50 (who still bellied up with a 51 share) and children (a 58). Locally, KNBC-TV, taking a page from KABC-TV’s post-Michael Jackson coverage, devoted nearly its entire 11 p.m. newscast to “Cheers” and was rewarded with a 27.8/53. The show itself pulled a 44.5/63 in Los Angeles, with the highest ratings coming in Minneapolis (54.8/72) and, appropriately, Boston (54.1/58). “The Tonight Show With Jay Leno,” tying into the “Cheers” mayhem, averaged a 16.1/43 in Nielsen overnight markets — the third-best rating ever for the NBC late night series, behind Johnny Carson’s final show and the 1969 on-air marriage of Tiny Tim and Miss Vicki. NBC, which was asking $ 650,000 for 30 -second spots during “Cheers,” averaged a 36.7/54 for the night, which started off with a 21.3/34 for “Seinfeld” and 39.6/56 for the 22-minute pre-show. ABC and CBS posted 12 and 11 shares, respectively.