WASHINGTON (CBS) -- A new study suggests that most Americans' eyes are bigger than their wallets when it comes to purchasing a new car. As a result, ignoring a vehicle's actual and long-term costs is putting a lot of consumers into an unnecessary financial vise.
The car affordability survey by Interest.com, a unit of financial research firm Bankrate, found that a median-income household in only one of the 25 largest U.S. cities -- Washington, D.C. -- could afford the $32,000 it takes to buy the average new car or light truck these days.
Mike Sante, Interest.com's managing editor, says the affordability study was created "to provide consumers with a a smart and simple way to decide how much they should really spend on a car before ever setting foot in a dealership."
The study took into account a variety of factors, including median incomes in the nation's 25 biggest metropolitan areas, as well as differences in sales taxes, insurance costs and other data. And the conclusion, according to Sante, is that many would-be car buyers have a poor sense of what that new car is costing them.
Without a proper financial plan, or a real notion of what a vehicle really costs, "you wind up putting all of your discretionary money into a car payment," Sante said. "And you wind up... in a situation where you're living paycheck to paycheck, and wondering how that happened."
Instead, Interest.com advocates what it calls the "20/4/10" rule when buying a car. That means having a down payment of at least 20 percent, financing that lasts no longer than four years and total monthly expenses for a car, including principal, interest and insurance, not exceeding 10 percent of a household's pre-tax income.
Using those metrics, Sante says, can ensure consumers aren't devoting too much of their income to transportation, "and [that] you're going to have some left over to invest in yourself, which is just a critical thing that people have got to do these days."
Auto industry observers say affordability is certainly an issue for car-
buyers, especially as prices have risen over the past several years.
"Consumers have been able to offset rising prices by taking advantage of longer term auto loans, low interest rates and leasing," noted Alex Gutierrez, a senior analyst at Kelley Blue Book. "While this has worked out well for the industry in the short term, if interest rates begin to rise or lending conditions become less favorable, affordability could become a significant hurdle for those thinking about buying a new car."
After housing costs, a car is one of the largest single purchases most Americans make. But Sante says it's an expense we can control.
"Certainly the... fact that pensions are going away, and it's a do-it-yourself retirement, you've got to be responsible for putting your kids through college," he said. "This is all sort of sinking in. And I think people are starting to ask more questions about what do I have to do to be able to save the kind of money that I need to be able to be saving, so that I can have some sort of sense of financial security."
© 2014 CBS Interactive Inc.. All Rights Reserved.