The following explanation was sourced from http://www.english-test.net/forum/ftopic9464.html and posted by Yankee on 12th June 2006:
If something is economical, it saves money or resources. It’s similar in meaning to “not wasteful” eg. “An economical car uses a minimum amount of gasoline (petrol) to drive a maximum number of miles (kilometers).”
The word economic is related to the economy or finances, money matters, or wealth eg. “There was a rise in unemployment due to the economic downturn.”
robertdesenior 