finance Minister P Chidambaram has asked banks to improve their operational efficiency to bring down interest rates. He was talking to SBI officials at its headquarters in Mumbai on Saturrday. He said, RBI recently lowered its key lending rates. Some banks have passed the benefit to borrowers, but many others have desisted, citing high cost of funds.Mr. Chidambaram said the government is confident to achieve economic growth closer to 5.5 per cent during 2012-13 fiscal.