Mexico is cinching in its economic belt - hard. But even as the four-month-old administration of President Miguel de la Madrid Hurtado begins to absorb its latest $1.7 billion loan - the first installment of a $5 billion package from 530 banks worldwide aimed at bailing Mexico out of its $85 billion debt - some observers wonder whether the belt is being tugged hard enough.
On one thing, however, many Mexicans agree. The nation's leadership appears to have undergone what a Western diplomat calls ''a sea change'' of attitude - less bluster and self-justification, more willingness to recognize and explain the severity of the problem.
''Austerity is no longer a defect, it's a virtue,'' says Angel Gurria, director general of public credit and the President's principal nuts-and-bolts debt manager.
In an interview with two US journalists, Mr. Gurria noted with understatement that ''the atmosphere is not one of gaiety and relaxation.'' But he added, ''Now we are being ordered (by the President) to explain'' and to make the government's financial dealings more ''transparent'' to an increasingly concerned public.
The details of Mexico's economic crisis, its worst since the 1920s, are bleak , indeed. Over the past year:
* Inflation, now clipping along at 110 percent, has more than doubled prices.
* The peso, which had traded at about 25 to the dollar since the mid-1970s, has fallen to 150 to the dollar - a six-fold drop that has priced foreign travel and imported goods out of reach of most Mexicans.
* The price of petroleum, Mexico's principal source of dollars during the heady oil boom years, has slid sharply. After the Organization of Petroleum Exporting Countries cut its oil prices earlier this month, the price of light Mexican crude fell to $28 a barrel from $32, and heavy dropped to $23.50 from $ 27.50.
* Economic growth, running at about 8.5 percent in 1981, fell below 1 percent in 1982.
* The nation's debt, run up by the high-spending policies of former President Jose Lopez Portillo, stood at 50.7 percent of its gross domestic product (GDP) - one of the highest levels in the Western world.
The situation has cast a pall of gloom and uncertainty over much of the nation's business community. But there was at least a glimmer of good news yesterday with the National Bank of Mexico's statement that the country ran up a exchange controls and severe restrictions on imports, however, and it is not certain how long the country can maintain such restrictions.
The administration's willingness to be a bit less secretive has also won some praise.
''We are all very optimistic,'' says a senior Mexican business executive somewhat hesitantly. Like most Mexicans, he refuses to speak for the record. He adds that so far he has heard ''rhetoric'' and is waiting to see ''effects.''
He is waiting, in particular, to see how the administration plans to repay, or indemnify, stockholders of the country's banks. Last Sept. 1, in a surprise move that still leaves the financial community stunned, Lopez Portillo nationalized the banks rather than risk a major collapse.
But Shari Rettig, managing editor of The News, an English-language daily, respects the new administration's initial caution. ''I rather approve of the fact that they're not trying to do everything yesterday,'' she says. Basically, she adds, these administrators are young - ''not the old shouting, screaming type, but a quieter, more thoughtful group.''
Some, however, wonder how great the change really is. Under Mexico's one-party system, President Lopez Portillo headed the Institutional Revolutionary Party (PRI, known as ''the Tree'') and handpicked Mr. de la Madrid as his successor.
Dr. T. Noel Osborn, director of the International University of Mexico, says that with each election the new administration is ''only a notch away'' from the previous one. He adds that until this government begins to publish detailed data on its fiscal situation - and lives up to anticorruption campaign promises by going after allegedly dishonest high officials in the former administration - it will be ''very easy to doubt'' its intentions.
That clamor to make the data public may explain the willingness of Mr. Gurria to speak freely. Despite Mexico's plans for repaying its debts, he noted that:
* Some $20 billion in debt will be ''restructured'' (with payments postponed or reduced) by Aug. 15.
* New lines of credit worth $2 billion will be used not to finance new job-creating developments but for ''maintenance imports'' - such as a $100 million credit guarantee announced last week to allow Pemex, the government-run oil company, to finance equipment.
* Oil production, running near capacity at 1.5 million barrels per day, may bring in about $15 billion this year.
* Both private-sector manufactured exports and tourism are ''picking up nicely'' due to the cheap peso, with the latter expected to bring in $1 billion this year.
* Imports have dropped from $24 billion in 1981 to between $14 billion and $ 15 billion in 1982 - which helps to stem the outflow of precious foreign currency.
Interest rates are also well below the 14.3 percent on which earlier 1983 budget estimates were based. A drop of 1 percent in interest rates saves the economy about $750 million a year - although every drop of $1 in the price of a barrel of oil loses $500 million.
Many economists, however, are waiting for another sign: the depth of the government's spending cuts. As part of a debt-bailout agreement with the International Monetary Fund last fall, Mexico agreed to reduce its budget deficit from 16.5 percent to 8.5 percent of its GDP. A Western diplomat who tracks the economy says Mexico is ''unlikely to reach'' that 8.5 figure because of falling oil prices.