Car manufacturers are clearly preparing for a future when people don’t buy many cars. Ford plans to launch its ride hailing service in 2021. GM just invested $500 million in Lyft. Tesla CEO Elon Musk’s “Master Plan” tells Tesla owners that in the future they will able to add their car to a shared fleet that will “generate income for you while you’re at work or on vacation.”

But if you can’t wait, you can still get someone else to pay for your car today. Car-sharing services for privately-owned vehicles are a small but increasingly popular way to pay off monthly payments, or turn a luxury vehicle like a Tesla Model S into an affordable purchase.

Car-sharing services are now used by more than 1.5 million people in North America who rented more than 22,018 vehicles in 2015, estimate researchers at the University of California Berkeley. The number of users is expected to top 4.4 million this year. In Europe, the market is even bigger. Car-sharing emerged in Europe during the 1940s and gained in popularity in the 1990s as services such as ZipCar began renting out cars by the hour in major cities. Enabled by smartphones, services such as Getaround and Turo (formerly RelayRides) now invite private owners to rent out their vehicles as well.

The services work by making privately owned cars available via a smartphone app. Drivers must pass a background check and then can rent out vehicles by the hour or day, usually at prices similar to or lower than a conventional rental car.

Turo and Getaround both told Quartz that a small but growing portion of their owners in the US are making car payments (or banking the equivalent value) by renting out cars about one week per month. Turo, which only rents by the day, and Getaround, which offers hourly as well, reported similar revenue numbers and payback periods.

Owners, on average, only needed to rent their cars out for about a week per month to earn enough to meet a standard payment for the models analyzed by Turo. Since the average car is parked about 92% of the time, owners typically have plenty of idle time to offer. One Turo user, Ryan Cwynar, said he bought a used $62,000 Tesla with a no-down-payment bank loan, and make enough renting his car out that it might end up being essentially free. “I have been driving a Tesla around for about half a year and to date I have paid nothing out of pocket for it,” Cwynar wrote by email.

Covering car payments isn’t everything. Cars may often be rented at peak times (although owners can choose when their vehicles are available). The car services acknowledge damage, accidents, and wear and tear may occur while others drive owners’ vehicles. But that doesn’t seem to have dampened enthusiasm for car-sharing services. And as both car-sharing and autonomous vehicle technology takes off, researchers are imagining a future of on-demand mobility at the push of an app.

“It is entirely conceivable that AVs will…come to dominate personal transportation by 2050, much as mobile phones have come to dominate personal telecommunications,” write University of California Berkeley researchers in a 2015 study. “Moreover, on-demand mobility as enabled by smartphone apps and changing generational expectations could…grow to occupy a sizable (though probably not majority) share of personal transportation by 2050.”