The U.S. economy grew at a modest 2.8 percent in the final three months of last year, the fastest growth in 2011. Americans spent more on cars and trucks, and companies built up their stockpiles. But growth in the October-December quarter — and all of 2011 — was held back by the biggest annual government spending cuts in four decades. The Commerce Department says the economy grew just 1.7 percent last year, roughly half of the growth in 2010 and the worst since the recession.