"Economic" is used to describe anything which has a relationship with the economy (money and trade) of a place or the finances that someone or a group of people have. Common uses include:
economic growth
economic development
economic reform
He advocated keeping wages high during periods of economic decline.
It is our plan to create jobs now and sustain economic growth for years to come.
He has a keen understanding of our economic system and knows how to create jobs.
"Economical", however, refers to avoiding waste: it usually means “thrifty” or “operating with little waste” (as in “the store brand is the economical choice”).
Electric lights are economical, clean, and give more light than gas.
This car drives like no other, and is more economical and cheaper to run.
This delicious, economical and simple family-style pudding serves 8 to 10 people.
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".