all the other solutions are great, but if you want to kill 2 birds with one stone i would suggest the following.
1. go to bucket list, far right column (world championships)
2. click on start button.
braking abs off
shifting auto, unless you like the manual
suggested line off
damage, fuel limited
class rest. open
this will give you +70% earning for cash
3.pick whatever make of car you are trying for
4. race each event. remember each race for each event has 2 heats so it will take awhile. but that is whats great about it.
for all 10 events you get
total : 1,871,700 xp, but its 2 heats so 3,743,400 xp. you only need 2,642,500 xp to reach level 50. not only will you get the level 50 but will be working on your bucket list as well. be forewarned it does take awhile to do them all thou