Forza Motorsport 5

Very simple to obtain. You need to come in 1st, 2nd, or 3rd in 50 of the 16 person career races. There are well over 400 races and you need that many golds to finish the achievements in this game so this will be something you unlock along the way.

If you are having difficulty, turn on the assists and play on new driver difficulty. You take a 10% credit penalty, but you will balance it out with the affinity level pretty quickly.