Throw in the drop in used-vehicle prices that's on the horizon, which will cut the value of trade-ins, an expected rise in interest rates and the possibility of a prolonged partial shutdown of the federal government, and today's strong sales look increasingly at risk.
Things are fine, for now. Edmunds.com predicts that the industry will sell in excess of 15.5 million new vehicles this year and hit 16.4 million next year.
To the extent that increase is built on an improving overall economy and jobs growth, all should be OK. But much of the market's strength today rests on easy credit.
Take auto loans. The average loan issued in the second quarter was for 65 months, up from 64 months in the same quarter last year, according to Experian Automotive.
Longer loans mean lower monthly payments. But they also mean that many consumers will still owe money on their trade-ins the next time they're ready to purchase a new vehicle.
Adam Jonas, an auto analyst at Morgan Stanley, says the market is increasingly turning "silly," with lenders offering what he calls "pulse" loans -- any car shopper with a pulse, gets a loan.
Similar trends are even more worrisome north of the border, says Kevin Williams, president of General Motors of Canada Ltd.
"Is it sustainable when a consumer base is leveraged as much as the Canadian consumer base is today?" Williams said to The Globe and Mail, a Toronto newspaper.
Canadians are on pace to buy 1.73 million new vehicles this year -- marking at least a 25-year high -- but that's in part due to buyers being offered interest-free loans as long as eight years. Williams worries that those loans could be causing Canadian vehicle sales to spike just as home sales did during the U.S. housing bubble, the paper reported.
Canada may or may not be the canary in the North American credit coal mine, but there are plenty of worrisome signs right here in the U.S. market.
Subprime loans -- loans to customers with a credit score below 680 using Experian's Scorex Plus scale -- are growing.
In the second quarter, customers with subprime credit scores accounted for 27.4 percent of new-vehicle loans, up from 25.4 percent a year earlier. For used vehicles, subprime loans accounted for 57.3 percent of loans, up from 56.5 percent a year earlier.
So far, delinquencies -- defined as loans that are three months or more overdue -- remain well below historical averages, and have not risen in line with the rise in subprime borrowing.
In fact, while the increase in subprime lending is a trend worth watching, it's not necessarily all bad, says Lacey Plache, chief economist at Edmunds.com.
Plache says that though a large number of auto buyers fall into the subprime credit category, they are not necessarily deadbeats. Many people could not pay their bills after losing their jobs during the recession, she says. Those people are back to work, have regrouped and are back in the game.
"On the consumer side, you have many years of austerity in terms of how to handle their finances and a lot of debt has been paid down on the household side," says Plache. "People are living more within their means."
Down the road, analysts worry that U.S. interest rates are likely to rise. In addition, rising production volumes could lead automakers to launch an incentive war to move the metal. A Morgan Stanley study found that carmakers will add 3.5 million units of North American capacity between 2011 and 2015, a 25 percent increase.
While the industry has avoided an incentive war so far, it has increasingly turned to another way of getting buyers into cars: leasing. Leasing represented 26 percent of new-vehicle sales in the United States though June, according to Edmunds.com data. That's up from 21 percent for all of 2010 and way up from the recession trough of 17 percent in 2009.
Leases, like longer loans, can lower payments and make buying a car or truck more affordable. The problem comes when they expire.
Used-car prices are poised to soften as more off-lease vehicles hit the market, and trade-ins rise in line with new-vehicle sales. That will put pressure on currently robust trade-in values, which have supported the rise in sales since the recession.
"The scariest thing about leasing is that it is a huge bet on used-car prices," Jonas says. "You're selling your cars today on the premise of used-car prices remaining where they are for a long time."
Tom Webb, Manheim chief economist, argues that there is room for lease volume to grow a "couple of percentage points" because leasing today is being "done right," unlike leasing in the late 1990s.
Back then, residuals were set too high and too many of vehicles that were lousy retail sellers were being leased to people with poor credit, Webb says.
Now, "these are the right residuals, the right car and the right customer," Webb noted during a conference call with analysts and reporters last week.
But he also said that the impending number of off-lease vehicles set to hit the market in 2016 has some auto company lessors "worried" because their dealer networks will not be able to absorb all of the volume. Yet those same companies continue to write new leases at a record pace.
"This is akin to the person who frets about their weight," Webb says, "while making their way to the all-you-can-eat buffet."