Alan Greenspan, economic adviser to former President Ford, yesterday called for an $18 billion to $20 billion tax cut beginning next January, but said it would come too late to help President Carter politically before the 1980 election.

At a meeting of the American Statistical Association, Greenspan said the tax cut should be aimed at improving the economy "in 1981 and beyond" on grounds that there no longer is anything that can be done this year and in 1980.

'THEY are confronting an impossible economic situation," Greenspan said of the Carter administration. "As far as I'm concerned, they ought to write off the next six to nine months and concentrate on bringing down inflation in 1981."

Greenspan also raised the possibility that if home mortgage interest rates keep rising, it could burst the current speculative boom in the housing market and send housing prices plummeting by 10 to 20 percent or so.

The GOP economist, now head of his own consulting firm, placed the odds of that happening at still "no more than one in four, and maybe less than that." But he said the possibility had grown over the past several weeks.

Greenspan's comments came as, separately, Otto Eckstein, former presidential economic adviser in the Johnson administration cautioned against cutting taxes in 1980 for fear that it would be inflationary.

Eckstein told a press conference at the same meeting "we do not believe we now have evidence that the recession will be severe enough to require a tax cut. The economy is going too strong. Operating levels are too high."

President Carter has raised the possibility that he may propose a tax cut if the recession worsens seriously, but has ordered aides not to discuss the issue lest they fuel speculation about shifts in policy.

Carter has been plagued by conflicting interpretations of his pronouncements in recent weeks. The official White House line is that the president still sees no need for a tax cut and wants to wait before deciding.

Meanwhile, Alfred E. Kahn, Carter's anti-inflation adviser, all but ruled out a major tax cut for 1980 and said the administration "will strongly resist any quick turnarounds" in spending and tax policy.

Kahn told an interviewer in Dallas that "that aren't any new major ideas under consideration." The White House has published a series of options for changes in its guidelines program and is talking with business and labor.

Greenspan's proposal for a tax cut centered primarily on a reduction in corporate income tax rates to help spur new business investment. He called for a gradual cut in corporate rates to 40 percent by 1985 from 46 percent now.

The Ford adviser also disputed suggestions by the administration that trimming Social Security payroll taxes would not exacerbate inflation, as some analysts fear cutting income taxes would.

Greenspan said for his part, the only tax cut that could be justified from an economic standpoint would be a tax cut for business. But he conceded that given political realities, two-thirds of the reduction would go to individuals.

Greenspan also:

Opposed the prospect of a federal loan guarantee for the ailing Chrysler Corp. as "a bad idea" that flies in the face of the "principles of the free-enterprise system."

Predicted that the jobless rate,now 5.7 percent of the work force, would rise to between 7.5 and 8 percent before peaking in mid-1980, but said the likelihood is that unemployment will not hit 8 percent or higher.

Greenspan said the recession was apt to be somewhat milder than predicted by administration economists in an unofficial forecast last month that allowing Chrysler to go under would hurt the economy. "The auto sales will go to other domestic producers," he said. "I worry far more about the . . . precedent that will be set." But, he noted, except for a decline in output, "we have not seen any meaningful downturn."

Forecast that interest rates soon wo