Two new inhuman comedies premiere on CBS tonight. They bear no relation to life as anyone knows it.

"Billy," the first, at 8 o'clock on Channel 9, is an Americanized and zoned-down version of the 1959 John Schlesinger film "Billy Liar," which was about a British lad who escaped his working-class trap by drifting or springing into fantasies and lies.

In the TV sit-com version, produced and directed by John Rich and written by Dick Clement and Ian La Frenais, Billy is a 19-year-old Pennsylvania kid who works in a mortuary. All the details of his life and environment have been so Cloroxed and Windexed that the point of the escape is lost; everything just barely exists, so nothing is to be gained by escaping into something that exists even less.

In addition, Billy is played by a great big gollywhomper named Steve Guttenberg, who got killed in the first or second reel of "The Boys From Brazil," Guttenberg is fresh and friendly and he radiates well-being, yet it is essential to the premise of "Billy" that he be something of a loser, or else why would he have to flee into fantasies? You take one look at Guttenberg and his like-me grin and find it impossible to believe that Billy's impossible dreams of glory really are impossible.

The rest of the cast is shabby and drab except perhaps for Roone Arledge look-alike Bruce Talkington as Billy's mortuary coworker, Millien. Fortuitously enough, Don Adams makes a welcome guest appearance in one of the fantasies (apparently the scene in which The Incredible Hulk drops by, shown repeatedly in promos for this show, is from a later episode).

Daydreams and fantasies have been part of many other TV shows, including last year's "James at 15." Billy at 19 doesn't justify reviving this gimmick -- certainly not to accommodate such jokes as, "She's called Virgin for short -- but not for long."

Like "Billy," the second new comedy, "Flatbush," at 8:30 on Channel 9, was shot on the too-clean streets of a Hollywood backlot, and if ever a show cried out for background detail, this one does. Particularly since there is virtually nothing in the foreground.

As for me, give me a female jiggle show over a male gaggle show any way. "Flatbush," a Lorimar production, is another boys-in-groups sitcom along the creaky lines of "Welcome Back, Kotter" and CBS's own miserable "Dukes of Hazzard." In fact they could have called this "Dukes of Flatbush"; it's about five cardboard-cutout Brooklyn buddies who get into scrapes and jams and never for a moment seem like inhabitants of this galaxy. Maybe "Battlestar Flatbush" would be a better title still.

A tape of the premiere supplied by CBS for preview came complete with an approximately 18-minute audio gap, during which no sound could be heard. This proved more of a blessing than a deprivation. When the sound came back, it was the familiar TV liturgy of squealing tires and shlock rock.

It seems the boys had broken up a gang of two car thieves and a not very riotous chase listlessly ensued. Thus does "Saturday Night Fever" meet "Smokey and the Bandit."

Playing the scrubbed-up pals, who call themselves the Fungos, are a pack of up-and-coming non-entities, of whom Sandy Helberg, doing a whiney and precious imitation of "Kotter's" already whiney Horshack character, is the most unquestionably insufferable. But Vincent Bufano, as Turtle, has the kind of attractive presence that sometimes occurs when a sensitive face is attached to a musclebound body, and he may go places -- the farther from Flatbush, the better for him. CAPTION:

Picture, Randy Stumpf and Olivia Barash, two of the stars of "Flatbush," a CBS-TV series premiering tonight.