Taken for a Ride?

Zipcar renters can cruise up to 180 miles a day, and the company will pick up the gas tab. Most don’t drive that far, but today’s pump prices make it an attractive prospect—so much so that Zipcar’s local membership has spiked 40 percent since last year. At the same time, the Cambridge-based firm’s growth has been stunted by the larger gas bill it’s footing. And if its drivers really began overusing their privileges, things would get tough. A worst-case scenario for Zipcar:

1. Bob, as we’ll call him, rents a Honda Element (23 miles per gallon, highway) for $11 an hour, when gas costs an average of $4.15 a gallon.
2. He drives roundtrip from Boston to Chicopee. Total: 180 miles, 7.8 gallons of fuel.
3. The trip takes 2½ hours. He pays $27.50 for the car; Zipcar pays $32.50 for gas.
4. Zipcar loses $5 on the trip. Bob ruins it for everyone. Thanks, Bob.