Dollar Thrifty Automotive sent a letter to Hertz and Avis seeking each company’s “best and final” proposals for a possible merger.

The proposals should be submitted by early October, Tulsa, Okla.-based Dollar Thrifty said in a statement.

“Continuing uncertainty is in no one’s best interest and both Hertz and Avis have the information they need to put forward their best and final offers,” Thrifty’s CEO Scott Thompson said.

Hertz is trying to thwart the planned combination of Avis and Dollar Thrifty after shareholders rejected an offer in September 2010. Dollar Thrifty recommended on June 6 that investors not accept Hertz’s second bid because it undervalues the company.

Avis and Hertz, meantime, separately have been seeking Federal Trade Commission approval for a deal to buy Dollar Thrifty, the fourth-largest US rental-car company.

In a letter to Avis CEO Ronald Nelson and Hertz Chairman Mark Frissora, Thompson wrote that those regulatory efforts have been successful, before setting the early October deadline.

Shares of Dollar Thrifty fell $2.46 to $61.91 last Friday. Avis fell 12 cents to $11.61, while Hertz declined 48 cents to $9.71.