After nearly six months on the job, Treasury Secretary G. William Miller remains in many ways a puzzle even to his close associates and administrative colleagues.

In his 5 1/2 months as Treasury secretary, Miller has oiled the administration's once-rusted economic policymaking machinery and gotten it running smoothly again. He's taken the lead on the federal bailout of Chrysler Corp., negotiated an accord with organized labor much needed politically by the Carter administration, and embraced his lobbying and politicking duties in a way to please the most partisan of President Carter's White House aides. And he has muffed more than a few shots as the administration's chief economic spokesman.

Through it all, he has remained so intensely a private man that his own thoughts and positions on issues often are impossible to surmise, other administration officials say.

With no sharp internal debate on many issues such as whether there should be a tax cut, and with others such as whether to propose a big new tax on gasoline or rationing left for another time, the Carter administration's latest round of economic decision making gave a few new clues about Miller.

As head of the Cabinet-level Economic Policy Group, Miller, the smooth former chairman of Textron, ran meetings crisply, pushing the group through relatively tight agendas covering a host of issues. But by some acounts, he offered few opinions of his own on the substance or politics of the issues.

At the end of a session, the Treasury chief would summarize methodically what the participants had decided and what remained to be done. As the give and take proceeded, "He never let on what his own views were," said one bemused observer. "In many areas, we still don't know what he thinks."

Miller came to the Treasury job abruptly and unexpectedly last August after Carter fired his predecessor, W. Michael Blumenthal, as part of a surprise Cabinet reshuffling. (Cabinet aides frequently had questioned Blumenthal's role as a team player.)

With financial markets in a tizzy both here and abroad in the wake of his mountain-top sojourn, Carter needed a quick replacement -- and Miller was a natural. He had been serving for six months as the Carter-appointed chairman of the Federal Reserve Board. He got along well with the White House staff. And he already had FBI clearance.

Since August, Miller's tenure has been a mixture of frenetic energy and enigmatic leadership, according to people in and out of government. Many observers believe that the new secretary's strongest suit so far has been the same driving administrative talent he displayed as chief executive officer of Textron. He is a manager, not a source of innovative policy ideas.

"The thing about Bill Miller is that he's smart, but shallow," says one former associate, a view shared widely by others who have worked with the new secretary. "He's fast on his feet and a skillful politician. But he's a 'don't bother me with the issues' sort of man."

A poised and sometimes-charming man, Miller, 54, is the quintessence of the corporate chief executive officer, with a flair for politics as well. Able to become the center of attraction in almost any crowd, he smiles easily, is good at small talk and rarely seems out of sorts in public.

But Treasury staffers say the new secretary quickly turns ice-cold if he's displeased about something, and he has no stomach for niceties such as lengthy discussions of issues or detailed briefing papers about complex subjects.

"You'd better get to him early if you want something because he makes his mind up quickly and it's awfully hard to change him after that," says one Treasury staffer who's had experience with the new secretary. "The word is that the guy who gets to him first has the advantage."

Miller demonstrated just how fast and how skillfully he can move only a few days after he took office. He astounded onlookers by quickly and deftly pushing through a delicate accord with labor on the wage-price guidelines program that had been stalemated for months -- a job insiders say Blumenthal was not likely to have accomplished.

Miller says he tackled the job even though labor relations is hardly part of Treasury's portfolio because "it fell to me as EPG chairman," and he's proud of the outcome so far. The accord, which was formal enough to be released with considerable fanfare as a printed document by President Carter at the White House, "took all the tension off and put the participants on a track of working together," Miller says.

Since then, Miller has continued to consult with labor leaders, as he puts it, "to make sure we are keeping our side of the bargain."

The Chrysler problem was awaiting on his desk when he first arrived, too.

In the sort of comment that has earned him a reputation as a hip-shooter in domestic affairs, he initially refused to countenance more than $750 million in aid for the nearly bankrupt auto maker, which had asked for $1 billion in loan guarantees. Not long after, he was forced to ask Congress to okay $1.5 billion, more than the company ever had sought.

"We had to cross that bridge and be honest about it. We had to tell people it will take more than we thought," Miller now declares. Originally, no one had any independent assessment of how much the company needed, and when the figures were available, that was that. Chrysler got the help, and Bill Miller had a bit of egg on his face.

Most recently, the secretary lobbied actively to help rush the long delayed crude-oil tax through the Senate. One administration official who had been consistently frustrated by Blumenthal's reluctance to do backroom politicking, coos, "This guy's a whizz. He takes a much more active role."

But Miller's reluctance to reveal the inner man certainly shows in his dealings on Capitol Hill. Says a key member of Congress who has worked closely with him: "You tend to feel about him a great deal like you feel about Jimmy Carter: You meet him, but you don't feel you know him."

The effectiveness of his lobbying isn't universally accepted, either. One key member deeply involved in the crude-oil tax declares flatly, "I don't see where he's done a thing."

Even where Miller's strength is greatest, there may be some offsetting losses, at least in the view of some officials. Although economic policymaking meetings unquestionably run more smoothly with Miller presiding than Blumenthal, insiders say the analysis accompanying discussion of key issues isn't as thorough as it used to be, with substance sometimes being sacrificed for the sake of meeting an agenda.

Moreover, colleagues say the new secretary still hasn't established the EPG as the kind of cohesive policymaking body that it was in previous administrations. "The president still hears too many voices on economic issues," complains one insider. "There isn't any underlying philosophy."

On the international side, Miller's performance so far gets distinctly mixed reviews. Many international bankers, for instance, complained bitterly about the policy positions he took while chairman of the Federal Reserve. Too easy on inflation, was the word. That dislike followed him to Treasury. t

When he attended the International Monetary Fund meeting in Belgrade last fall, the foreign press was full of stories about bankers complaining about Miller and his rah-rah speech extolling the virtues of American economic policies.

Yet most foreign finance ministeries, like that in West Germany, for instance, are quite satisfied with U.S. policy at the moment despite the high rate of inflation here. The Germans are impressed particularly that despite all the trouble in Iran and Afghanistan, and the wild gyrations in the price of gold, the dollar has remained strong enough against the German mark that the German central bank has not had to intervene lately to prop up the dollar.

The secretary's other foray into international economics -- a trip to Saudi Arabia to confer with that nation's government on the touchy issue of oil prices and production -- almost ended in disaster. Not only did Miller return empty-handed, but he nearly created an incident as well.

The gaffe came at an oil ministry picture-taking session in Riyadh when Miller, as a stunt, publicly presented Saudi oil minister Sheik Ahmed Zaki Yamani with a home-made spin-the-dial game that pointed to a hold-prices-down or increase-oil-production slogan each time the needle stopped on a square.

Those who saw the incident say Yamani clearly looked uncomfortable with the ploy, laughing nervously at first, mostly out of politeness.But Miller persistently nudged him to spin the needle repeatedly -- "Spin it again, Mr. Minister."

Not only did the display violate Saudi etiquette, but Yamani was not in a game-playing mood: There were reports -- confirmed by at least some officials later -- that several of Yamani's relatives were among the hostages in the grand mosque takeover that week. The sheik was getting reports every 15 minutes.

It is in his role as chief economic spokesman for the administration, however, that in many ways Miller has done least well.

In early November, for example, with all signs pointing to a further economic slowdown in 1980 -- indeed, that was and is the administration's economic forecast -- the secretary declared blithely that the recession was then "half over." He later retracted the statement after reminders of just what the forecast encompassed.

The few speeches Miller has made have been routine and filled with rhetoric. "What gets me about this guy is that he gives you the same programmed stuff in private that he says on the stump," says one veteran observer. "You really begin asking whether there's anything more there."

More recently, Miller has been much less visible. That fact, combined with the dramatic Oct. 6 credit-tightening moves by the Federal Reserve Board, has left an impression with many people that the administration has virtually left the running of the economy to the Fed's current chairman, Paul Volcker, who has had a much higher profile.

Last week a Miller statement to the effect the United States planned to make no gold sales at the present time may have been a boner. Instead of keeping the market guessing as to the Treasury's intentions, he removed all doubt -- hardly a good move with gold caught in the midst of a speculative boom.

Whatever his overall rating as a Treasury secretary, however, there is one point on which everyone agrees: Miller is politically ambitious. He impresses almost everyone as seeking a still higher office once his term as secretary is over, but no one can seem to get a handle on it. (The secretary's office already has denied that he's angling for the vice president's job -- an earlier rumor.)

Miller likely will face his toughest tests in the coming months. To begin with, there are the policy problems posed by a sharply rising defense budget in the face of a predicted economic slowdown and continuing inflation -- all tough enough dilemmas for any would-be policymaker.

Then, too, Miller has served notice that he plans to depart from the tradition of previous Treasury secretaries and go out on the campaign stump for Carter this year. Balancing that chore with his other responsibilities will be no small task this year.