Relief from high inflation rates expected to hold through '85.

As the US government prepares to release June's consumer price index Tuesday, several economists say that over the next two or three years, inflation will be less pronounced than in the recent past.

Some acceleration in prices is expected as the recovery matures and excess production capacity diminishes.

But ''fundamental changes in the economy have improved the long-term inflation trend considerably,''

says Ben E. Laden, chief economist at T. Rowe Price Associates.

Among the changes he and other economists cite: deregulation of key industries like trucking, greater competition from imports, and a loss of union power, which has helped feed a trend toward smaller wage gains. These shifts could lead to inflation being less severe for the next two or three years than has been the case at comparable points in recent recoveries.

Food prices give one indication of how well-behaved inflation has been. Nationally, retail food prices fell at a 1.5 percent seasonally adjusted annual rate for the three months ended in May, government figures show.

''We would expect food prices to level out'' and rise 4 to 5 percent overall in 1984, says Donald Straszheim, vice-president of Wharton Econometric Forecasting Associates.

The upbeat outlook on inflation has significant political implications. One of Walter F. Mondale's key tactics will be to try to convince voters that President Reagan has mismanaged the economy. Making that charge stick will be tougher when one of the economic conditions the average voter feels most keenly - inflation - is behaving surprisingly well.

Overall, prices for June will have moved up ''0.3 percent at the worst,'' or about the same pace as in May, says Robert Gough, economist at Data Resources, Inc. (DRI), a major forecasting firm. If such a rate persisted, it would produce an annual inflation rate of less than 4 percent.

Other economic indicators also are moving in the President's direction. The government announced Friday that personal income rose 0.8 percent in June, twice the May increase. And with inflation at low levels, the increase in income is not just offsetting higher prices but also is adding to consumer spending power.

Strong growth of consumer income is one reason economists now expect the gross national product figures for the April to June period - scheduled for release today - to outpace the Commerce Department's preliminary estimate of 5.7 percent at an annual rate.

That the economy is expanding without strong upward pressure on prices is also evident in the producer price index, which tracks the cost of goods at the wholesale level. Over the 12 months ended in June it rose 2.2 percent. In the 12 months ended in May, the price of all consumer goods rose 4.2 percent, about one-third the pace of inflation in 1980.

Forecasters offer a variety of reasons for this moderate rate of inflation. Sharper competition, prompted by deregulation of US industries, has forced firms to cut costs and hold down wages, Mr. Laden notes. Then too, the strong US dollar has led to a flood of imports, which puts price pressure on US firms.

And as long as the strong dollar persists, US consumers in effect are able to call on excess production capacity overseas, thus reducing the pressure dwindling excess production capacity in the US normally would put on prices, Mr. Straszheim notes.

Also playing a role in holding down prices, economists say, are a more disciplined monetary policy and lower expectation of inflation. Such expectations often become self-fulfilling as consumers hoard goods and cause shortages that push up prices.

''Inflationary expectations seem to be lessening, especially as the US monetary authority quickly tightens whenever there is a surge in economic growth or inflation,'' notes Allen Sinai, chief economist at Shearson Lehman/American Express.

A faster rate of growth in business investment in better machinery and the fatter productivity gains such investments help make possible also are important , economists say. Productivity is the amount a worker can produce in one hour.

As a result of these forces, Mr. Laden says, the inflation rate in the 1985- 87 period will peak at only about half the 15-percent rate seen in 1979.

Other forecasters are even more optimistic. DRI sees consumer prices rising 5 percent in 1984, about the same pace in 1985, and 5.5 to 5.75 percent in 1986, according to DRI economist Gough.

There are dissenters from this relatively rosy view. Due to strong employment gains, rapid economic growth, and other factors, the US will face ''a 7 percent inflation rate around the end of this year and near 8.5 percent by year-end 1985 ,'' argues Citibank economist Henry P. Korytkowski.

And even the 5 percent inflation rate Gough expects is still well above what Americans had experienced before the economically turbulent 1970s.

Meanwhile, the Republican and Democratic candidates continue to disagree on another key pocketbook issue - whether a tax increase will be needed after the election to trim the federal deficit. Over the weekend Mr. Mondale renewed the charge made in his acceptance speech that the administration had ''a secret plan that they will announce after the campaign,'' to raise taxes.

Presidential spokesman Larry Speakes said Friday, ''there is no secret plan to raise taxes.'' While refusing to rule out a tax increase, Mr. Speakes quoted the President as saying new taxes should not be considered ''until we get every last dollar possible'' out of federal spending.