Low interest rates, an aging auto fleet and a continued rebound in housing likely fueled strong December sales, capping another good year for the industry. And a fiscal cliff deal averting a middle class tax hike is giving auto shares a turbo boost. Year-end data from automakers comes Thursday. But economists surveyed by Bloomberg expect a 9.8% bump in December. That would bring the annualized sales rate for the month to 15.2 million units.