Stock index futures pointed to a flat open on Tuesday as investors awaited key economic data for new signs of recovery one day after the Dow and the S&P rallied to their highest closes in more than 15 months.

Investors will get a better picture of the state of consumer spending after automakers report U.S. vehicle sales for December. Economists in a Reuters survey expect median annualized domestic sales of 3.90 million cars and 4.45 million trucks, versus 3.87 million cars and 4.44 million trucks sold in November.

The Commerce Department is to report November factory orders at 10 a.m. EST. Economists see a rise of 0.5 percent, compared with a 0.6 percent gain in the prior month.

We had a very strong day yesterday, and it will be interesting to see whether the momentum will continue ... Basically, it will depend on the data today, including car sales, factory orders and home sales, said Peter Cardillo, chief market economist at Avalon Partners in New York.

Kraft Foods Inc sold its North American frozen pizza business for $3.7 billion to Swiss food giant Nestle AG and sweetened its offer for Britain's Cadbury Plc .

Early Tuesday, Berkshire Hathaway Inc said it voted no to Kraft's proposal to issue 370 million shares to facilitate the deal. Kraft rose 2.7 percent at $28.18 in premarket trading.

Merger and acquisitions always increase enthusiasm among investors, as they generally indicate better days ahead for the economy, Cardillo said.

S&P 500 futures were unchanged and in line with fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures were unchanged, while Nasdaq 100 futures lost 2.25 points.

The National Association of Realtors will release pending home sales for November at 10 a.m. EST. Economists look for a 2.0 percent decline, compared with a 3.7 percent rise in the previous month.

RadioShack Corp shares gained 6 percent to $20.96 after Goldman Sachs upgraded the stock. Tenet Healthcare Corp rose 7 percent to $5.83 after Goldman added the stock to its conviction buy list.

Google Inc is expected to unveil its highly anticipated smartphone in its first foray into an increasingly crowded market that could shape the future of the company's advertising business.

Apple Inc is set to release a new tablet device later this month, the Wall Street Journal reported Monday, as anticipation builds about its next big product launch since the iPhone.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)

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