Standard & Poor's Ratings Services raised its ratings for Dollar Thrifty Automotive Group Inc., citing improved operating and financial performance by the two car rental companies.

The firm boosted Dollar Thrifty's ratings to "B" from "B-," and planned to keep the company's ratings under review for a possible upgrade due to Dollar Thrifty's proposed buyout by rival Avis Budget Group Inc.

Dollar Thrifty has been working with Avis to get antitrust clearance for the possible takeover bid. Regulatory approval is expected early next year, Snyder said.

S&P also raised the rating on Dollar Thrifty's senior secured credit facility to "B+" from "B-" and the recovery rating to "2" from "3", citing the reduction of the term loan.

The Tulsa, Okla.-based Dollar Thrifty has seen its operating and financial performance improve since last year, amid stronger demand, higher prices and cost cuts. The company has reduced $1.1 billion in debt off its books since late 2008.

Dollar Thrifty's adjusted operating margin jumped to 20.6 percent in the 12 months ended in September. That's up from 6.8 percent in the prior-year period.

Snyder said she anticipates Dollar Thrifty will continue to report improved results, even if the company doesn't end up being acquired by Avis Budget Group.