Car sharing is changing. Sunil Paul, the clean-tech entrepreneur behind a new San Francisco service called Spride Share, looked at the numbers and decided that 10,000 shared cars in the U.S. was not likely to change the world. The answer, Mr. Paul says, is personal car sharing, in which car owners can get modest payments of $5 to $8 an hour to allow their often-idle car or truck to be shared, either by friends and neighbors or the larger car-sharing community.