Early in July, Valero Energy in Texas got the unwelcome news that Mexico would be cutting supplies to one of the company's Gulf Coast refineries by up to 15 percent. Mexico's state-owned oil enterprise is one of Valero's main sources of crude, but oil output from Mexican fields, including the giant Cantarell field, is drying up. Mexican sales of crude oil to the United States have plunged to their lowest level in more than a dozen years.

The same week, India's Tata Motors announced it was expanding its plans to begin producing a new $2,500 "people's car" called the Nano in the fall. The company hopes that by making automobiles affordable for people in India and elsewhere, it could eventually sell 1 million of them a year.

Together the developments were emblematic of the larger forces of supply and demand that have sent world oil prices bursting through one record level after another. And while the cost of crude has surged before, this oil shock is different. There is little prospect that drivers will ever again see gas prices retreat to the levels they enjoyed for much of the last generation.

Unlike the two short, sharp oil jolts of the 1970s, the latest run-up has been accelerating over several years as ample supplies of crude oil have proven elusive and the thirst for petroleum products has grown. The average price of a barrel of oil produced by the Organization of the Petroleum Exporting Countries doubled from 2001 to 2005, doubled again by March this year and jumped as much as 40 percent more after that.

For American motorists, a full tank of gas costs nearly twice what it did at the start of last year, racing past the $4-a-gallon mark, and has begun cutting into other household spending.

"What can you do? You need gas," said Barry Modeste, a construction worker who stopped his van at a Shell station in Takoma Park, Md., one recent morning to add $15 worth. It was enough, he said, to get him to a cheaper station nearby.

"If you don't have gas, you can't get to work. And if you can't get to work, you don't get paid. And if you don't get paid, you can't buy food. We're at their mercy."

Last month, 51 percent of the respondents in a Washington Post poll said rising gas prices were causing a serious financial hardship for them or others in their household. It was the first time a majority had said that since the poll began asking that question eight years ago.

The rising prices are also adding to inflation, aggravating the U.S. trade deficit — oil now accounts for about half of it — and taking a toll on businesses already struggling with the economic slowdown caused by the housing and financial crises.

Abroad, riots shook India after the government trimmed fuel subsidies. Truckers in Britain, France, Spain and South Korea have clogged roads to protest fuel prices. In the Philippines, soaring prices for oil and petroleum-based fertilizer have derailed the economy and ignited calls for a cut in the tax on oil imports.

Even after oil prices have tumbled more than $24 in the past two weeks, largely as the result of easing tensions in the Middle East and slowing U.S. economic activity, crude is still trading near historic highs.

STEADY SQUEEZE

Earlier oil shocks have had obvious causes. In October 1973, OPEC raised prices and declared an oil embargo against the United States and other countries that had supported Israel in its war earlier that month against its Arab neighbors. The embargo ended in March 1974, but pricing power had shifted from the oil companies to the producing countries. In 1979, prices soared again after the Iranian Revolution curtailed output and consumers and oil companies went on a spree of panic buying.

Now, however, there is no one culprit and no single international crisis to blame. Instead, world demand has been increasing faster than supply, steadily squeezing oil markets.

This in turn has signaled to investors that prices are inevitably heading higher. Financial players, such as Wall Street banks and hedge funds, have bet just that, investing tens of billions of dollars in oil futures. Critics on Capitol Hill and elsewhere say this speculation has turbo-charged the market, helping lift prices even more.

The tightening of the oil market reflects decisions made a decade ago, when conditions looked radically different. Regular unleaded gas was less than a dollar a gallon. Oil was little more than $10 a barrel. And the Economist magazine, predicting prices could soon be half that, ran a cover story with the headline: "Drowning in Oil."

Those low prices sent the wrong signals to consumers and oil companies alike.

Demand for oil jumped as U.S. sales of gas-guzzling cars soared and China's breakneck economic expansion picked up pace.

The low prices of the late 1990s also dampened the impetus for finding new supplies. Oil companies delayed exploration for new fields. Capital spending dropped 15 percent at the biggest oil companies in late 1998 and plunged as much as 70 percent at the smaller ones. Too few drilling rigs were built. And refineries weren't expanded or upgraded, making it hard for them to use the lower-quality crude oils that have become a larger portion of supplies or to produce the right balance of products as gasoline use is stagnating and diesel fuel use growing.

Investment slackened just as finding new supplies was becoming more difficult and costly. Most of the world's big, easy-to-tap fields have already been discovered and largely drained.

Some analysts argue that peak oil production has already been reached. Others say the peak remains a ways off but perhaps not very far. Though capital spending by big oil companies has again picked up pace in the past couple of years, spurred by higher prices, exploration is still falling short.

"It's not that we're going to run out of oil or hydrocarbons, but it's not going to become available as fast as uninhibited, unrestricted demand," said Sadad Husseini, a consultant and former petroleum geologist at Saudi Aramco.

Just two decades ago, the world could pump 15 percent more oil than it needed. Today, that spare production capacity has practically vanished — it's now about 2 percent beyond the world's total daily consumption of 85.5 million barrels. That makes the market sensitive to rumors about anything that might endanger existing production.

At the beginning of the pipeline, high oil prices have been a gusher of good news.

SWIMMING IN PROFIT

Any company that owns oil in the ground or a share of what's pumped out of it is swimming in profit. Exxon Mobil, the biggest of the independent oil giants, last year broke records for U.S. corporate profits, chalking up $40.6 billion. This year, it is on track to earn even more.

Thanks to the rapid and sustained rise in prices, oil-producing countries are also accumulating vast reservoirs of money in one of the most massive transfers of wealth in history. Every day, oil consumers pay $6 billion to $7.5 billion more for crude oil than they paid six years ago. At the current rate, they will pump more than $1.5 trillion a year into the coffers of OPEC, Russia and other oil exporting countries.

Some Middle Eastern countries are already on a shopping spree: indoor ski facilities on the edge of the desert, water-borne hotel complexes, new industrial cities.

A century after Henry Ford's Model T revolutionized American life, Tata's Nano could do the same for India. Unveiling his company's concept for the car early this year, Chairman Ratan Tata placed the Nano in a narrative of technological endeavors that led from bicycle to jet.

But in an era of scarce oil, the Nano could take the world down a rough and costly road.