Next iPhone impulse buy: Your car

NEW YORK (CNNMoney.com) -- Here's a smartphone tool that people prone to midnight shopping binges should handle with care.

On Aug. 4, car shoppers will be able to get a new car paid, financed and insured using only an iPhone.

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This is what the USAA iPhone app will look like after Aug. 4.

Already, members of USAA -- the insurance company for military servicepeople, veterans or their family -- can research new cars and get price quotes from up to three area car dealers, using either the website or iPhone application. Now, just about everything but driving the car off the lot can be taken care of without ever walking into the dealership.

The USAA online and iPhone car-buying services are run by Zag.com, a company that specializes in creating car-shopping interfaces for various companies and organizations.

But experts, even those at USAA, caution that car shoppers should be wary of completely bypassing the dealership.

While websites and iPhone apps can give you specifications and even images of the cars, they can't replace the all-important test drive.

Once a car shopper has narrowed the field of choices down to two or three possible models, it's important to get the behind the wheel.

"You can hear all the greatest things about the car in the world," said Karl Brauer, senior analyst with the automotive website Edmunds.com, "but all those people aren't built exactly like you and they don't weigh the same as you."

There are also issues of simple personal taste and some of these things only become apparent when comparing one car to another from the driver's seat.

Waiting until just before signing to test drive the one car you chose only makes it harder to say no if it isn't quite right for you.

"At that point, you're too psychologically committed to critically analyze the car in person," Brauer said.

For now, the USAA iPhone app doesn't offer the same detailed ability to choose options and amenities as the company's website does, but it will by the end of the year, USAA says.

Although using the USAA app, and other phone-based tools like it, have some risks, they could be helpful at the car dealership, said Jeff Bartlett, an editor for ConsumerReports.com. Unlike a paper print-out from a website, an interactive phone app allows you to keep researching new options even as you're talking to the salesman or, for that matter, any time you think of it.

Financing is another feature that, ideally, should be tried out at the dealership. While USAA makes getting financing convenient, the dealership could still offer better terms.

Tom Ferries, who leads USAA's product integration team, recommends using the one-click auto financing tool as a competitive weapon at the dealership. Either the dealer offers to beat the USAA rate your iPhone is offering or you press the button and the dealership doesn't get your financing business.