Forza Motorsport 5

This achievement will unlock after completing your 10th Championship Series in the career. You do not need to earn a gold medal on any race, although if you are following the road map you should have gold medals on all of your races as you move through the game. If you complete the leagues and championships in order, this achievement will unlock after completing the following Championship Series:

Super Mini (Sport Compact League, 8 races)
Modern Sportscars (Sport League, 8 races)
Iconic Sportscars (Sport League, 8 races)
Coupster (Sport League, 8 races)
Executive Sport (Sport League, 8 races)
Note that you do not need to complete the bonus races included with the Championship Series for this achievement to unlock.