March 27 (Reuters) - Chevrolet's advertising will be handled by Commonwealth, a new joint venture of Goodby, Silverstein & Partners, an Omnicom company, and McCann Erickson Worldwide, an Interpublic company.

The new ad agency and the naming earlier this year of Carat as General Motors Co's agency for its media planning and buying operations is part of the U.S. automaker's effort to cut costs by $2 billion over five years and more effectively build Chevy into a global brand.

Commonwealth, based in Detroit, will handle development of creative campaigns globally. GM is staying with Goodby and McCann, its two largest ad agencies, which handled 60 percent to 70 percent of the markets Chevy is in, a spokesman said.

"These agency consolidations are expected to create about $2 billion in savings over the next five years, with a portion used to take advantage of key global marketing opportunities and strengthen the focus on our global Chevrolet brand, and a portion hitting the bottom line," GM global marketing chief Joel Ewanick said in a statement.

GM's advertising expenses last year totaled almost $4.5 billion, according to the company's annual report. WPP -owned ad-tracking firm Kantar Media estimates GM spent $1.83 billion on U.S. ads last year with almost 60 percent of that used on Chevy, which makes up about 70 percent of GM's U.S. sales.

Earlier this month, GM executives told analysts at a dinner meeting in New York the company expected to save $2 billion over five years from the consolidation of advertising agencies globally.

Commonwealth will be led by an eight-person board with assignments handled through global hubs in Detroit, Milan, Mumbai and Sao Paulo, GM said.

Up to now, Goodby has led the Chevrolet creative account in the United States, the brand's largest market, while McCann has been Chevrolet's agency of record in several global markets, including Mexico, Canada, Brazil, India, Japan, China and Latin America.

Chevy, which previously used 70 global agencies to handle its accounts, will still use McCann in China and India, and other agencies in Uzbekistan, GM said.

In January, GM awarded its global advertising media planning and buying account to Carat, a unit of media group Aegis Media . Carat had been GM's European agency of record since 2007 while Starcom MediaVest, a unit of the Publicis Groupe , handled GM's U.S. account for more than a decade.

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