The economy, which was sluggish through 1985 and still is running in low gear in the first half of 1986, will pick up speed by the end of the year and move at an even faster pace through much of 1987, according to an economic forecast presented here today by the Business Council.

Real gross national product -- the value of goods and services produced and provided in this country -- will rise by 2.7 percent in 1986 and 3.2 percent in 1987 compared with 2.2 percent in 1985, according to the forecast.

"This latest outlook represents another turn toward increased optimism" about the nation's economic performance, said the council, a national-policy advisory group composed of chief executive officers of large U.S. corporations.

"The reasons for the anticipated improvement lie in the expected stimulus from lower interest rates, a lower dollar and lower inflation," the council said in its report.

But part of what the council called a "relatively pleasant" economic picture for the next two years was based on a now-questionable assumption -- that there would be "no tax reform bill in 1986."

That speculation was upset Wednesday when the Senate Finance Committee unanimously approved a sweeping proposal to rewrite the federal tax code. The committee's 20-to-0 vote, coupled with the business community's initial, generally favorable reaction to the bill, could be enough to send it through Congress by the end of 1986, business leaders said here.

But the executives said they do not believe at this point that a tax bill would significantly alter their economic predictions.

Despite the council's rosy view of the economy, some gremlins out there could cause things to go awry, and two of the most mischievous are rising installment debt and dwindling consumer confidence, said Willard C. Butcher, chairman of Chase Manhattan Bank.

"Inflation rates in 1986 look to be the lowest since the early 1960s," rising an overall 2.9 percent in the current calendar year and by 3.6 percent in 1987, the council said.