Forza Motorsport 4

in reality, this achievement will come will natural progression through season mode, however you can also compete in online races for a good expreience boost.

around 1.4 million xp is required to reach level 50 (credit to J1MMEH G for pointing that out)

you can re-play any race from the event list should you wish to earn some extra credits during season mode.

nb : hiring i driver is a good way to go through career mode, however you will only earn a small amount of XP towards driver level/affinity.

thanks to 'mfeforever' for adding : 'When self driving a race you are awarded the listed first place credits as XP (for both driver and manufacturer affinity levels) and some number of actual credits (based on race finish multiplied by the bonus for the difficulty setting levels.) When using a hired driver, you receive 50% of the calculated prize credits and no XP for driver or manufacturer affinity level. However, most races have a fixed or random bonus award in credits, driver level XP, or manufacturer affinity level XP. When using a hired driver, you receive the amount of the bonus award for the event as though you had self driven it.
Bonus amounts for driver level or manufacturer affinity stated as a percentage (e.g., driver level + 50%), do not award any experience points. As a hired driver earns neither type of experience points any percentage bonus applied to 0 will result in 0 bonus points. Of course, credit bonuses work fine since there are credits awarded. Further, this means a mystery bonus will sometimes give experience points (e.g., affinity + 2000) or sometime not (e.g., affinity + 75%.) Hope this helps!'

**if you do feel the need to vote negatively, could you at least give a reason so i can improve this?