WASHINGTON – U.S. factories boosted output last month, and December ended up being their best month of growth in five years.

Strong auto sales and growing business investment in machinery and other equipment are keeping factories busy and helping the economy grow.

The Federal Reserve said Wednesday that manufacturing production increased 0.7 percent in January. And output soared 1.5 percent in December, according to an upward revision. That’s the biggest one-month gain since December 2006.

Overall industrial production, which includes output by mines and utilities as well as factories, was unchanged in January. Still, the flat reading was mostly because Americans used less energy to heat their homes during an unseasonably warm winter.

Jonathan Basile, an economist at Credit Suisse, said December and January marked the best two months of growth for manufacturing since the summer of 2009, when the recession ended.

Basile also pointed to a regional survey conducted in February by the Federal Reserve Bank of New York released Wednesday that showed factory activity in that region grew for the third straight month. That suggests the strong momentum in January carried over into February.

And even though a measure of hiring in the New York survey dipped, it still indicated more hiring ahead.

“It looks like we’re set up for a little faster growth in the coming months,” Basile said.

Factory output has risen 16.7 percent from its low point during the recession, in June 2009, according to the Fed’s industrial production report. It is still 7.1 percent below its December 2007 peak.

In January, auto production climbed 6.8 percent – the biggest gain since July 2010. That coincides with best growth in car sales in more than two years.

Industrial machinery production increased 2.2 percent, after an even larger gain in December. Computer and electronics production moved up 1.4 percent.