Costco Wholesale Corporation reported strong sales gains in Dec. thanks to rising gas prices and appreciation of foreign currencies against the dollar.

The warehouse club said net sales rose 11% to $8.26 billion for the five-week period ended Jan. 3, 2010 and 8% to $24.93 billion for the 18 weeks ended Jan. 3, 2010 compared to the similar period last year. That exceeded expectations of 7.9% forecast for the month by analysts polled by Thomson Reuters.

Much of the growth came from overseas stores, where comp store sales surged 25% during the five week period compared to 5% in the U.S. After backing out gas sales, comp store sales rose just 2% in the U.S. during both periods.