If you use economic, you're saying it is related to economics.
If you use economical, you're implying that it's economically efficient.
So, you could say:
The gas mileage was economical.
Meaning: not a lot of gas is burned in the vehicle
If you said:
The gas mileage is economic.
Meaning: You'd be describing that gas mileage is an economic identity and that it could be rated economically. However, you're not saying that it is economical; you're just saying that it has an effect on the economics relating to the vehicle.
If that confused you, here's a two more examples:
The economic crash that occurred due to the recession was not as bad as people thought.
The economical toilette saved me a lot of money, due to it using less water.
Although, not many people would look at a toilette in that way :P. I just can't think of any other examples, haha.