Welcome to the Zipcar Low-Car Diet Get ready to trim down with the Low-Car Diet Challenge! That is, trim down expenses, pollution, and congestion. With gas prices at an all-time high, it's a great time to start. Here's how it works. You agree to keep your car keys in your junk drawer for just one month, from July 21st through August 15th. In return, you get a loaded public transit pass, a free 1-year Zipcar membership, some driving time and more goodies that we'll tell you about a littl...