Improved sales last month haven't shaken a growing pessimism among the nation's restaurant owners, which has pushed their economic outlook to a record low, the National Restaurant Association reported this week.

Led by stronger same-store sales, the Washington D.C.-based trade group's monthly restaurant performance index improved for the second straight month in May, rising 0.2 percent from April.

Yet despite the gains, most owners continue to expect weaker sales in the months ahead, with fewer investing in new equipment or expansion plans, the report said.

"Only 12 percent of restaurant operators expect economic conditions to improve in six months," Hudson Riehle, the trade group's senior vice president, said in a statement. That's the lowest reading for owner expectations in the six-year history of the index, Riehle said.