If you check in the WordReference dictionary, you'll find that economical means avoiding waste; thrifty; producing or resulting in profit or savings. Not the same as economic (of or relating to the production, distribution, and use of income, wealth, and goods and services; of or relating to the science of economics).
I don't buy into the Random House definition that economical = economic. In Britain that is considered a solecism.
Economical can be used instead of economic (especially in writing), but it is normally used in the sense of not wasting resources.
Example: This is an economical solution to the problem (it saves money).