Zipcar Inc., the company that rents cars by the hour or day, fell to a record low after reducing its forecast for sales this year as the pace of new members slowed.

Zipcar dropped 37 percent to $6.75 at the close for the biggest drop and the lowest price since the company’s initial public offering in April 2011. The shares had declined 21 percent this year through Thursday.

Membership grew by 21 percent to 731,000 members in the second quarter, lower than the Cambridge company expected, chief executive Scott Griffith said. A radio advertising campaign was costly and ineffective and Zipcar dropped the medium from its marketing for the rest of the year, Griffith said.

Zipcar said sales this year will rise to $272 million to $278 million, compared with an April forecast of up to $296 million. Analysts predicted $290.8 million, the average of estimates compiled by Bloomberg.