DAVID LETTERMAN, NOTORIOUS lead foot, must have sensed he had a kindred speedster on his show last June when he asked Lucille Treganowan, "What's the fastest you've driven?" Caught off guard, the 66-year-old grandmother came clean. "Without thinking," she recalls, "I said 120."

Thus did Treganowan win over Letterman, just as she has charmed Oprah Winfrey, the folks at Today, Dateline, Extra and just about anyone who ever walked into her two auto-repair shops, in Penn Hills and Pittsburgh. What has propelled her onto the talk-show circuit is Lucille's Car Care, a lively repair manual and auto primer, published in June and now in its second printing. This week she makes her sitcom debut, playing herself on Tim Allen's Home Improvement.

Exactly how a former bookkeeper became an authority on lug nuts is a study in serendipity and elbow grease. Divorced in 1960 after 10 years of marriage, she was left to raise three children, then ages 6, 3 and 18 months, by herself. Her job search got nowhere, until she drove her battered 1946 Plymouth into Scuro's Auto Repair shop in Penn Hills and overheard the owner say he had just fired a mechanic whose wife was the bookkeeper. "I said, 'Hey, I can do that,' " says Treganowan, then 30. "I was in the right place at the right time."

Hired to do paperwork, she studied repair manuals at night, eventually winning a promotion to mechanic in 1962. "The boys in the shop were like, 'Oh, go back to the office. What do you know?' " remembers Treganowan, who knew enough to start her own shop, Transmissions by Lucille, in 1973. Two years ago she began hosting a cable show, Lucille's Car Care Clinic, for the Home & Garden Network.

Her 12-hour days leave little time for hobbies, though Treganowan likes to plant petunias in old transmissions and decorate her shops with them. "Almost everything I do," she says, "is related to the automotive industry."