The index of US leading indicators increased in December, extending its rise to four straight months, as the world’s largest economy expanded. The Conference Board’s index, a gauge of the outlook for the next three to six months, increased 0.5 percent in December, after a revised 0.4% gain in November, the New York-based group said Saturday. The median forecast of 49 economists surveyed by Bloomberg called for a 0.4% advance. An improving job market and plunging gas prices continue to support consumer spending that makes up almost 70% of the economy.