Helmut Kohl can smoke his pipe again. For eight wearying weeks on a campaign trail dotted by drafty convention halls and factory entrances, West Germany's newly elected chancellor left his tobacco at home rather than risk weakening his rumbly voice by smoking.

Now a smiling, confident Kohl greets a visitor to his office with pipe in hand and a prediction that U.S.-West German relations are about to take a dramatic upswing. U.S. praise for Kohl's strong victory over the Social Democrats last Sunday suggests that Washington also foresees a new era of sustained good feeling.

Kohl's tone and answers in a 90-minute conversation indicate that he will be amiable where his predecessor, Helmut Schmidt, was abrasive. Kohl will be general, vague even, on points of disagreement that Schmidt sought to make specific. Unlike Schmidt, Kohl will be prepared to go along to get along, his advisers say.

But the conversation also leaves the impression that Kohl's amiable, rumpled manner is a cloak for a steel-hard determination and an instinct for political survival that his rivals and opponents in West German politics have consistently, and disastrously, underestimated over a decade.

For all the outward warmth and optimism generated by Kohl's impressive personal victory on a platform of support for U.S. missile deployment here, he is unlikely to be a docile partner for President Reagan's administration when German and European interests collide with U.S. designs.

Early tests that could dampen the initial warm glow and pull Kohl into Schmidt-like postures include the missiles issue, continuing U.S. sanctions against Poland, East-West trade and finance and protectionist measures in world trade.

"Kohl has the confidence of Washington, and he can perhaps persuade Reagan to protect American interests in a way that also protects European interests," one of the chancellor's political associates said, "and Kohl will try to get more understanding from Reagan that the Soviets also have interests that have to be taken into account."

Kohl, who has held every important office in his Christian Democratic Union, is enthusiastic about this role of explaining the Old World to his new friends in Washington. He will be sounding for them a message of optimism and regeneration in a West Germany that will be reminded of the large American role in reconstruction.

"For German-American relations, irrespective of missiles and all those things, last Sunday was an important day," Kohl said. "I did not bring it about, it was already there, but I was a sort of a catalyst for a fundamental change.

"My opponents committed a stupid error by suggesting to the electorate that I was sort of the American in Germany. That is not a label that harms you. On the contrary, German interests can be represented only in and with the alliance."

"I remind the young people of who helped us after the war," he said. "I tell them of the days when I was going to school. I was 15 or 16 . . . and we were half-starved, and there were the Americans who came along with their trucks, and they gave us something to eat. It is bad that the Americans have not made more use of this capital. The Soviets didn't do anything, but they are carrying out a big psychological offensive."

"What Americans can and should mean to Germans has been systematically lowered" during the 13 years of Social Democratic rule. "Not that Schmidt wanted it, but he didn't have the strength to steer his party, where you found a number of people who said that it's not good for us to place ourselves in a corner when the storms of history pass over our country. That is wrong. You can see on any map that we are not Austria."

In detailed conversations as well as on the campaign stump, Kohl relies on simple formulations and homey anecdotes that border on being corny, but that proved popular with German voters this year. Measuring the amount of calculation and guile that goes into his air of simplicity will be a major preoccupation for foreign governments now.

Kohl, portrayed by associates as a man who feels keenly Germany's responsibility for the World War II ravaging of Poland, the Soviet Union and the rest of Europe, is likely to stress to the Reagan administration that U.S. sanctions against Poland only add to human suffering and help entrench the present regime.

He has already begun to prod Washington to probe more vigorously for a compromise solution on medium-range nuclear missiles, and he has made it clear that he will resist efforts to make new restrictions on trade with the Soviets a major topic at this year's seven-nation economic summit.

Kohl's attachment to the ideal of eventual German reunification and to bringing West European states closer together in the short term has not received much attention in Washington, but it occupied a major place in his thoughts yesterday.

"I am after all the leader of the largest single democratic political party in Europe, and I am much closer to the political leaders of Europe personally" than was the aloof Schmidt. "This European crew, which wants the unification of Europe, has very good personal relations. . . . For me there is no such thing as a policy of either-or when it comes to Europe and the United States," he concluded.