I frequently see one of these used when the other is required.

Problem:
The adjectives “economic” and “economical” are best not treated as synonyms.

Explanation:
The most common definitions of the adjective “economic” are:

• related to the production and use of wealth, commodities, and income;
• related to the science of economics; and,
• related to an economy.

An example of the proper use of the adjective “economic” is “The President of the U.S. vetoed the bill for economic reasons.”

The adjective “economical” means thrifty.

An example of the proper use of the adjective “economical” is “You should buy a more economical car.”

I believe that the tendency of some people to use “economic” where “economical” is required is consistent with my “Devolution toward Simpler” linguistic hypothesis.

For example, it is simpler to say “an economic use of gasoline” than it is to say “an economical use of gasoline”, which requires an extra syllable.

Solution:
Use “economic” when referring to income, wealth, commodities, economics, or the economy. Use “economical” when referring to thriftiness.