Forza Motorsport 4

The best way to earn money and affinity is the same as TP x NeOnZz´s way, but on a different track. On Testrack layout A you can cut the corners (like you can see in the video below). You can do a map in like 20 seconds x 50 = 1000 seconds = 16 min 40 sec. Also difficulty can be adjusted for more money and affinity, so make sure you don't turn on anything unnecessary (Like the race-line or automatic transmission).
Note: the video is taken from Forza motorsport 3, but the track is exactly the same as in forza 4.
Edit: you need to download the launch track pack first to select the testrack maps. I got a code for this trackpack within the copy of the game, but you can download it on xblm as well.
Edit: Go to assists and put driving on normal or simulation. If you put driving on assisted you will be driving against invisible walls.

The actual amount of affinity xp that's needed for the achievement is 2,642,500 (strange number)

I would recomend doing this in the eventlist so you don't need to do extra races
with some special settings and the #9 Peugeot 908,
under Assists/Difficulty choose:
Class Restriction: Open (this will allow you to enter races with R1 car instead of the original F,E,D,C,B,A,R3,R2
Yes this will give you -25% bonus but just turn off some assists and you are on plus bonus again.

Also, doing the world championchips with a R1 car won't hurt
(you can enter all of them with the R1 car with class restriction to open) :D

Hope this helped and comment before giving a negative vote if you think I'm wrong :)

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
steering simulation
stability on
traction on
shifting auto, unless you like the manual
suggested line off
damage, fuel limited
rewind on
opponent professional
upgrades upgraded
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
F: 125,900
E: 135,800
D: 144,000
C: 157,000
B: 197,000
A: 204,000
S: 209,000
R3: 221,000
R2: 236,000
R1:242,000
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

If you only go for affinity points (that's what this achievement is about right :-)) and are too stingy to buy a dlc, you can do the following:

- select Peugeot R1 998
- go to eventlist
- press start and adjust restriction to 'open'. As Mr Razero mentions, this cost you -25%, but bonusses are NOT taken into account for affinity points!( You might choose professional opponents so you won't have to pass them twice)
- choose 'multi-class-event E class' (5th block 2nd row 2nd column), it will get you +100% affinity.
- Choose Bernese Alps clubcircuit (1st race) for 17.400 affinity points in 4:40 (62 per sec) or the easier Road Atlanta (3rd race) for 15.000 affinity points in about 4:20 (58 per sec).

It's still a lot of boring racing (12 hrs) but it seems faster then 50k in 16:40 (50 per sec) :-)

An easy (but boring) way to do this is to go
Community - Race online - Create match - Settings

Sedona speedway
12 Players
11 AI
Professional
R1
50 Laps

And give your self the Peugeot with 998 rating and the bots the Chevey with the 910 rating

You should come 1st by a lonbg shot and every race is 22ish mins long and nets you about 55,000 XP for both Driver level and affinity

Xx Corndawg xX has told me it is just over 2.5 Million XP

WLG Shepard's Solution looks faster if you have the lauch track pack (i dont as its my brothers game but its downloadable from XBLM) use the Test Track Layout A (For more info watch his vid, im not going to repost the vid as it would be pointless)

This is not a solution but just a chart showing the exp required in between each level for anyone else that was curios. I did not create this i found it here:

http://www.codeleakers.com/showthread.php?37088-Forza-Motors...


Affinity Level XP

Level..........Reward..........Xp to Next Level
0...............0..................2500
1...............1000.............6500
2...............2000.............7000
3...............4000.............8500
4...............7000.............10500
5...............15000...........13000
6...............25000...........16000
7...............35000...........21000
8 ..............45000...........27000
9...............55000...........34000
10.............65000...........40500
11.............75000...........46900
12.............75000...........53000
13.............75000...........59600
14.............75000...........66000
15.............85000...........73500
16.............85000...........84500
17.............85000...........88500
18.............85000...........106500
19.............85000...........125000
20.............95000...........135000
21.............95000...........137500
22.............95000...........102500
23.............95000...........77500
24.............95000...........50000
25.............105000.........50000
26.............105000.........50000
27.............105000.........50000
28.............105000.........50000
29.............105000.........50000
30.............115000.........50000
31.............115000.........50000
32.............115000.........50000
33.............115000.........50000
34.............115000.........50000
35.............125000.........50000
36.............125000.........50000
37.............125000.........50000
38.............125000.........50000
39.............125000.........50000
40.............135000.........50000
41.............135000.........50000
42.............135000.........50000
43.............135000.........50000
44.............135000.........50000
45.............145000.........50000
46.............145000.........50000
47.............145000.........50000
48.............145000.........50000
49.............145000.........50000
50.............150000.........0

Totals........4729000.......2642500