Economic is all about how money works, but something economical is a good deal. You might take an economic studiesclass to understand the ebb and flow of cash in the world, but if you buy a used textbook for it, you're being economical.

Economic appeared in the late 1500s, referring to household management, but its sense of relating to a country's wealth first appeared in the 1800s. It's still related to economics (the study of the transfer of wealth) or economy (a country's wealth), but not thrifty (that's the other one). Here are some economic examples:

The Fed said economic conditions will likely warrant "exceptionally low" interest rates through at least mid-2013. (Business Week)

Some slowing is expected in 2012 because of global economic woes. (New York Times)

The word economical also showed up in the 1500s, referring to household management, but it refers to being thrifty or not wasteful, which is still the definition today:

Not long after The New York Times profiled an inventive and economical restaurant experiment taking place in Windsor Terrace, Brooklyn, the experiment ended. (New York Times)

Electric lights are economical, clean, and give more light than gas. (Rose Buhlig)

If you want an adjective related to the economy, economic is your word. If you want a word to describe something that saves money, like buying used textbooks, use economical.