By DEXTER FORD

HOW far can a modern car really go? Given the increasing age of vehicles on American roads, we may be on the verge of finding out.

As a stubborn recession made drivers wary of new purchases for several years, the average age of vehicles on the road in the United States stretched to a record 11.1 years in 2011, according to the research firm R. L. Polk, which tracks vehicle sales and registrations.

Multiply that number of years by the annual miles driven — the E.P.A. uses 15,000 for the cost calculation on fuel economy labels — and it becomes evident that one pearl of conventional wisdom has become outdated.

In the 1960s and ’70s, when odometers typically registered no more than 99,999 miles before returning to all zeros, the idea of keeping a car for more than 100,000 miles was the automotive equivalent of driving on thin ice. You could try it, but you’d better be prepared to swim.

But today, as more owners drive their vehicles farther, some are learning that the imagined limits of vehicular endurance may not be real limits at all. Several factors have aligned to make pushing a car farther much more realistic.

Cars that have survived for a million miles or more have been widely documented, of course, but those tend to be exceptional cases. What’s different, and far more common, today are the online classified ads offering secondhand Hondas, Toyotas and Volvos with 150,000 or 200,000 miles — or more — not as parts donors but as vehicles with some useful life left.

One driver who has firsthand experience with this new paradigm of durability is Mark Webber, a 57-year-old Porsche salesman.

Mr. Webber has a full grasp of powerful new sports cars — in January he was in Southern California for sales training and track time with the 2013 Porsche 911 — but for his 35-mile commute to Herb Chambers Porsche in Boston, from Scituate, Mass., he drives a 1990 Volvo 740 with over 300,000 miles.

“I just can’t see the point of spending a lot of money driving a newer, racier car every day in city traffic when my old Volvo just wants to keep on going,” Mr. Webber said. “I guess you could say I’m just a New England tightwad.”

In Mr. Webber’s case, the enabler of his thrift may be global competition — and the Environmental Protection Agency.

Customer satisfaction surveys show cars having fewer and fewer problems with each passing year. Much of this improvement is a result of intense global competition — a carmaker simply can’t allow its products to leak oil, break down or wear out prematurely.

But another, less obvious factor has been the government-mandated push for lower emissions.

“The California Air Resources Board and the E.P.A. have been very focused on making sure that catalytic converters perform within 96 percent of their original capability at 100,000 miles,” said Jagadish Sorab, technical leader for engine design at Ford Motor. “Because of this, we needed to reduce the amount of oil being used by the engine to reduce the oil reaching the catalysts.

“Fifteen years ago, piston rings would show perhaps 50 microns of wear over the useful life of a vehicle,” Mr. Sorab said, referring to the engine part responsible for sealing combustion in the cylinder. “Today, it is less than 10 microns. As a benchmark, a human hair is 200 microns thick.

“Materials are much better,” Mr. Sorab continued. “We can use very durable, diamondlike carbon finishes to prevent wear. We have tested our newest breed of EcoBoost engines, in our F-150 pickup, for 250,000 miles. When we tear the engines down, we cannot see any evidence of wear.”

Dr. George Akerlof, who shared the 2001 Nobel Memorial Prize in economic science with Michael Spence and Joseph Stiglitz, may have predicted this trend of owners keeping cars longer.

In his 1970 paper, “The Market for Lemons: Quality Uncertainty and the Market Mechanism,” he noted that the prospective buyer of a used car knows far less about that car than its seller, a phenomenon he called asymmetrical information.

He reasoned that a buyer must assume that any used car is a lemon, because he has no way of knowing for sure whether that car has been abused or crashed. So a buyer, on average, is only willing to pay a below-average price. But the owner of a good used car knows that it is reliable, that he has driven it carefully, changed the oil regularly and maintained the car well.

If the owner of that good used car is not offered what he thinks it is worth, he won’t sell it, Dr. Akerlof reasoned. The overall quality of used cars goes down, because only below-average cars are offered, which drives down the value of used cars as a whole. The market is, in economics terms, broken, because the best cars in it are undervalued and are seldom offered for sale.

Because new car sales and leases dropped during the downturn, there are fewer of those to go around. So used-car prices have been rising over the last few years as recession-shy — and economics-savvy — drivers have pushed their cars ever farther.

With the value of some older cars actually rising, many drivers decided to drive their current car until it wore out. And because of the improving overall quality of today’s automobiles, many are discovering that it is entirely possible for a driver to wear out long before his or her automobile.

J. D. Power customer surveys of problems with new and three-year-old cars have shown, over the years, that cars are simply getting better. And that many of the problems that do surface involve new electronic gadgets like navigation, audio and phone systems that new owners may have a hard time understanding.

The trend toward better, longer-lasting cars seems to have begun way back in the ’60s, when the first imports from Asia started to encroach on American and European carmakers’ sales figures.

Another factor is that cars from the ’60s and ’70s were susceptible to rust and corrosion — many literally fell apart before their engines and transmission wore out. But advances in corrosion protection, some propelled by government requirements for anticorrosion warranties, have greatly reduced that problem.

“Competition is part of it,” said Peter Egan, a former auto mechanic and now editor at large of Road & Track magazine. “Japanese cars kind of upped everyone’s game a bit. With some exceptions, the engines would go a long time without burning oil or having other major problems.”

Hyundai and Kia, the South Korean carmakers, now include 100,000-mile/10-year warranties on their cars’ powertrains. If a relatively abusive driver can count on no major mechanical failures before 100,000 miles, a careful owner can — and does — expect his car to go much farther.

“With a ’66 Chevy Impala, you could rebuild everything on the car with a reasonable level of mechanical skill,” said Mr. Egan of Road & Track. “My parents, with their ’56 Buick, used to have the wheel bearings repacked with grease before a long trip. Nobody does that anymore. The lubricants are better, the machining is better.”

Some drivers, of course, keep driving their cars simply because they love them.

Edmund Willcox Clarke Jr., a Superior Court Judge of Los Angeles County, has driven his 1989 Porsche 911 Carrera for over 300,000 miles.

“It was my first brand-new car,” he said. “It was designed the way I wanted it, and looked and drove exactly as I wanted it. I loved it from the beginning, and I’m still fond of it.”

But Judge Clarke has lavished plenty of attention on his Porsche.

“I rebuilt the engine a couple of times, because of wear and tear and to keep the performance up,” he said. “I had it painted a couple of times to keep it looking stylish. My body fit into that seat and the car felt like an extension of me. I could tell when it was tired.

“I think the car was well made — that was one reason it lasted so long,” he said. “I stayed ahead of it. As long as it was reliable and loyal to me, I was loyal to it.”

Judge Clarke finally bought a new car last year, an electric Tesla Roadster that he can drive in the carpool lane on the clogged Harbor Freeway every weekday. But his 1989 Porsche still sits in his Manhattan Beach, Calif., driveway, carefully covered, as if waiting for its owner to return.