The 車 in Chinese chess is pronounced jū. Why isn't it pronounced chē?
Is this pronunciation used outside of Chinese chess?
It is an ancient pronunciation of that Chinese character. Because Chinese chess has been created for more than 2000 years. During this period of time, pronunciations of many characters have changed, "車" is one of them. But because this game has been widely spread, this ancient pronunciation has been retained.
This pronunciation is only used in this game.