Forza Motorsport 3

You unlock this achievement when you have around 1,670,000 XP.
How to get experience points?
- Complete races in Season Play
- Complete races in your Event List
- Complete races in Multiplayer. Make sure you cross the finish line. If you didn't finish the race you won't receive credits/XP.

Obviously you will get more XP for a higher finishing position. If you're like me and start off with Season Play without completing races in your Event List or in Multiplayer, you will get this achievement somewhere in year 5. Just keep winning races and you should be fine. If you're having a hard time winning races, adjust your difficulty.

Keep in mind YOU have to drive the car to gain XP. If you're hiring a driver, you will get credits but no XP.

I found this achievement takes a long time, but roughly between year 5 to 6 in season you should get this, my advice do the longer races for more money, which also gets you more experience points (XP), apart from that all I can say is do the longer races and keep winning you get more XP if you come in first.