In the business world, the outspoken ways of J. Peter Grace have been well known since 1945 when, at 32, he was installed by his ailing father as president of W.R. Grace & Co. But it has only been in the past few years that the irrepressible Grace, now the company's chairman, has gone public.

That began in 1978 when Grace was getting an annual check-up, and his doctor asked him to address a gathering of physicians at the University Club.

Grace says he was casting about for a subject when he read that President Jimmy Carter was planning to boost capital gains taxes.

"It was quite clear that President Carter didn't understand what makes this system work," Grace recalled in an interview in the executive suite atop the company's headquarters here. Grace made his speech "about taxes and what it is doing to the private enterprise system."

"We had a tremendous reaction," says Grace, who tends to use the editorial "we" in referring to himself.

Soon Grace was sending his message on capital gains taxes to each congressman. And sure enough, Congress this year reduced the tax on capital gains from 28 to 20 percent--an initiative for which Grace personally takes a measure of credit.

"Numbers don't lie," Grace states emphatically. The executive uses statistics the way a debater uses the language, in personal exchanges and in an amphitheater where members of the corporate board of directors gather before a huge screen to view the chairman's figures.

An extraordinarily energetic and proudly pugnacious 68-year-old, Grace can disarm a stranger by calling him "baby" and "dearie" and is bluntly inquisitive about people. "What religion are you?" he recently asked a visitor, followed by: "Irish descent?" "How many years married?" "Have you got children in school?"

With his success on taxes, Grace put a small team of his corporation's public relations people and economists to work issuing statistical attacks on the antibusiness bias of the govenrment and media. Grace's views, with statistical bolstering, appear in advertisements as well as corporate house publications and the company's annual report.

One advertisement, for example, showed how in 1978-1979 the percentage of return on total capital was lower among oil companies than among various big media concerns. That statistical presentation earned Grace an invitation to appear on the Phil Donohue television show.

More recently, Grace, a registered Democrat, has become the most outspoken supporter of President Reagan's economic program. Not long ago he used his statistics--"80,000 numbers"--and his corporate resources to challenge a Washington Post editorial critical of the Reagan tax plan, in a column and in advertising space he purchased.