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Tahra Makinson-Sanders of San Francisco expects to net between $3 and $5 an hour by renting out her car when she's not using it. Makinson-Sanders lends her 2006 Subaru Outback through Spride Share, a personal car-sharing network that, after a pilot period, began full operations in San Francisco in January.

It's an innovative way to help pay for the high cost of owning a car in the Bay Area, including her $250-a-month parking spot. "We're all looking at our budgets more closely," says the 42-year-old photography studio executive. "Every little bit helps."

Through a partnership with City CarShare, Spride Share cars can become available to more than 13,000 screened City CarShare members once hardware is installed that tracks usage and gives renters access via an electronic key fob. Owners designate when their car is available. Rental rates depend on the car: economy, premium, specialty, luxury. No beaters, please.

A Toyota Highlander SUV might command $9 an hour, while a late-model Honda Civic could rent for about $7 an hour, says Spride Share CEO Sunil Paul. Owners clear roughly 40% of rental rates. Spride Share estimates that sedan owners renting their car out 15 hours a week might typically earn $2,200 a year; premium vehicle owners, $2,800.

A different critter than Zipcar

The individual owner renting to the individual customer is how Spride Share and its brethren differ from Zipcar, which, with 500,000 members and cars in 28 states and Great Britain, is the dominant force in the car-share business. Instead, Zipcar has about 8,000 company-owned vehicles it parks in neighborhoods and rents out to nearby members.

Another neighbor-to-neighbor network, RelayRides, has been up and running since June in Cambridge, Mass. Owners set their own rates (Relay suggests $8 an hour for, say, a late-model Corolla, up to around $12 for an SUV.) Relay gets 15% off the top, then 20% for insurance.

Relay marketing director Boris Mordkovich says owners made about $250, on average, one recent month, while Relay's top earners brought in more than $600. Relay is signing up about 200 new borrowers a month in the Boston metro area, and launched in San Francisco in December.

Getaround, another San Francisco outfit, is testing its service in other California cities. All of the companies envision eventual expansion to major metro areas such as New York, Chicago and Washington. The trend is already global, with WhipCar operating in London and Communauto in Canada.

Insurance is key

The essential ingredient in a peer-to-peer rental fleet is the proper insurance. Spride, RelayRides and Getaround provide supplemental insurance -- through national carriers -- that covers the car and owner in case of an accident, up to $1 million per incident with RelayRides and Spride Share. Renters are covered, too, after a $500 deductible.

In most cases, you don't even have to own your vehicle outright, meaning loan-free, to rent it out. Most lenders don't object, although lease agreements occasionally nix subleasing. And, yes, the income generated is taxable.

Is it for you?

Of course, you have to get over the fear of lending your car to strangers. But renters who become members of the car-sharing network are screened, and their driving histories are checked. Penalties are assessed if cars come back late or dirty. Feedback systems allow owners and borrowers to rate other borrowers. Still, some car owners choose to rent only to friends and family.

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The wear and tear on your car might be less than you think. Renters typically drive just five or six miles for every rented hour. Some 80% to 85% of RelayRides rentals last from two to four hours.

Drivers mostly run errands: picking someone up at the airport, or making runs to Ikea, Costco or a grocery store, says Mordkovich. "I like the idea that my car might get used more often by people who can't afford a car," says Spride Share car owner Makinson-Sanders. And a little extra in her own piggy bank doesn't hurt.

The article was reported by Anne Kates Smith for Kiplinger's Personal Finance magazine.