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 9:41 a.m. in New York, after falling to $6.50 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 yesterday.

Membership grew by 21 percent to 731,000 members in the second quarter, lower than the Cambridge-based company expected, chief executive Scott Griffith said on a conference call yesterday after the markets closed. 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 as much as $296 million. Analysts predicted $290.8 million, the average of estimates compiled by Bloomberg.

The radio ad expenses contributed to a 27 percent increase in Zipcar’s acquisition cost per member, to $89. The company is offering monthly memberships, instead of annual ones, in some test markets, Griffith said.