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We rented a Model S from this company in Houston when we went to visit family in Texas for a week last year. It was absolutely fantastic to be able to drive a familiar, safe car. We would do it again in a heartbeat.
I got the impression that they were initially the owner of a Model S, decided to rent it out occasionally, the business took off, and they bought several more cars solely to rent out.
Probably helps that it's harder to do test drives at Tesla Galleries in Texas than many other states, so that expands the demand there.
The owners were super friendly and real Tesla enthusiasts. They might be willing to share their experience with you if you're serious about starting up a business doing the same.