There's the old saying that you can tell your true friends from acquaintances, because only your true friends will drive you to the airport.

But Shri Ganeshram, a co-founder and chief technology officer of Flightcar, definitely disagrees.

Flightcar hopes to disrupt the typically miserable car rental process at airports by pairing consumers' normally parked, unused cars with drivers who need wheels when they fly into U.S. airports. Their gimmick is to use valets to pick you up and drop you off at the airport in the car you're renting. The company has 10 employees.

The concept puts FlightCar, which rolled out its service at San Francisco International Airport in February, squarely in the category of start-ups trying to make something of the new "share economy", which has been pretty much defined by Airbnb for lodging, and now includes a number of ambitious competitors who want to apply the same concept to cars.

Ganeshram, who's only 18, co-founded Flightcar with two buddies, Rujul Zaparde and Kevin Petrovic, who are also both 18, in 2012.

Ganeshram was at the Massachusetts Institute of Technology when a friend introduced him to Zaparde and Petrovic, who were roommates at Princeton. They were all fascinated by the Airbnb concept, but Petrovic, who had a previous electronics business that required him to travel a lot, was intrigued by fixing the car rental process at airports.

Last May, they all took a break from school to work on Flightcar in San Francisco.

"The three of us decided to go to San Francisco, because that is where the sharing economy is big and where Silicon Valley is," Ganeshram says.

And they've been pretty busy learning and earning their chops. In October 2012, they got a $20,000 grant from Brandery in Cincinnati, Ohio to learn about branding and marketing.

"It set us up to get further financing, and it connected us with lots of investors, and people noticed us because of the legitimacy of the program," Ganeshram says.

Next stop: a stint with the start-up accelerator Y Combinator starting in December, and then $600,000 from individual investors as well as angel firm SV Angel, to test the concept.

Since February, Flightcar has matched 150 cars with 250 renters who wanted wheels at SFO.

In the share car rental space, they have some pretty tough competition though. Relayrides has a similar concept that operates in major cities throughout the country. So does Getaround. And in the airport market specifically, Silvercar, co-founded by former Zipcar chief technology officer Luke Schneider, also just launched its service in the Dallas-Fort Worth airport. Its cars are all shiny, new Audis that renters unlock using their smart phones, and then simply drive away.

"I love what Flightcar is doing, like Silvercar they are disrupting an old industry that has not seen innovation in a long time," says Tom Ball, general partner of Austin Ventures, in Austin, Texas, which led a series A investment of $11.5 million in Silvercar in the fall. Ball says the potential market for both companies is huge.

The entire car rental market in the U.S. is worth about $30 billion, according to research done by venture capitalists interested in the industry. Of that, Ball estimates about $8 billion is for car rentals at the 50 major U.S. airports.

But some industry experts question Flightcar's revenue potential, particularly because it has to spilt earnings with the car owners, and because it needs a fleet of valets to make the system work, as well as insurance for three parties.

"The cost of having to get the cars back to the airport is challenging, and what if the flight is delayed or late?" says Josh Stein, managing director of Draper Fisher Jurvetson, of Menlo Park, California.

There are also other nagging questions, such as what type of car consumers will get when they arrive, and whether the company can scale adequately should demand suddenly spike, Ball and Stein say.

Still, Ganeshram and his co-founders are young men with a plan. After they prove their concept in San Francisco, they hope they'll get more funding to introduce the service to other top airports including Boston, Chicago, Los Angeles, and Seattle.

"Our plan is to saturate the market, that is what we are aiming for here," Ganeshram says.