There are no rules, just different meanings, so you'll have to look them up in the dictionary. In the case of these pairs, the '-al' tends to mean ' like', so that 'economical', meaning efficient with or saving of money, is 'like economics', which is the study of production and consumption of goods and services, etc but I'd wager that the relationship does not hold through all the pairs of '-ic' and '-cal' words.
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