Fewer Americans are rushing to file their federal tax returns this year, but those who do are finding that the average refund has increased by almost 20 percent from 1987, the Internal Revenue Service said yesterday.

During the first five weeks of the year, the IRS received 9.14 million returns, down 17.5 percent from the 11.08 million filed in same period a year ago. The average refund was $656, up from $548 a year ago.

These returns, due by April 15, are the first since most of the new tax law enacted in 1986 took effect. And although some errors are turning up, the IRS said, there have been no big surprises.

"Early indications are that the filing season is going pretty much as expected," said Robert LeBaube, director of the IRS taxpayer-service division. The drop in the number of early returns is not significant, he said.

In fact, the number of taxpayers who delay filing has been growing each year. The figure for the first five weeks in 1987, for example, was down almost 8 percent from the previous year.

LeBaube said some of the procrastination this year can be traced to taxpayers' efforts to educate themselves about the new law. He noted that the agency has received 30 percent more requests for forms and explanatory publications than in any previous year.

The average taxpayer received a 2.2 percent tax cut from the new law. The fact that the average refund so far is almost 20 percent indicates that many taxpayers are having more withheld from paychecks than they owe, thus giving the government interest-free use of their money.

All taxpayers were required to file new W-4 forms with their employers before Oct. 1 to ensure that enough was withheld to cover their liabilities under the revised law. As in years past, about 80 percent of taxpayers can expect refunds. In the first five weeks of 1988, refunds were certified for 528,000 couples and individuals whose returns had been processed, up 3 percent from last year.

One IRS goal this year is to improve the accuracy of answers to taxpayers' telephoned questions. Past surveys by Congress' General Accounting Office found that accurate replies were given less than 80 percent of the time.

The IRS is testing the system. Agency officials pose as taxpayers and call in questions to see how IRS "assisters" respond. It is too early to point to any major findings, LeBaube said, "but we already have found some problems."

A spot check of early returns has turned up four problems areas, LeBaube said. They are:

On forms 1040A or 1040EZ, where many persons, especially students with part-time jobs, are failing to check a box indicating whether they can be claimed as a dependent by another person. Those returns are not being processed, he said, until the agency decides whether they should be sent back to the filers to correct that mistake.

On Schedule A, where some who itemize deductions are concluding -- erroneously -- that they are no longer allowed to deduct state and local income taxes and property taxes. The new law eliminated the deduction for sales taxes, not income and property taxes.

Those over 65 or blind, who are unaware that they still get a special tax break if they do not itemize, even though they no longer get an extra exemption.

Schedule D, on which capital gains are reported, is causing special problems because the law was changed so significantly in this area.