Go to freeplay and Select Indy Oval setup. Setup a 20 lap race, have there be 15 drivatars and put them on max difficultly, Turn off all assists because you won't need them. The Lotus is so fast on this course you won't even really need to do work. This method is kind of boring, but very calming. I'm probably going to do it alot this weekend so I can take advantage of my 1/2 off car prices.
If you drive 20 Laps it should take less than 15 minutes and you should get 21000xp and 21000Cr per race, which means you should level once or twice which will make around 63000Cr on top of the 21000. (31500 for one level, Non Vips only earn 15k per level). Your 21kCr with the additional 100% to 145% for the extra difficultly/affinity bonus will make you around 45000Cr. The % depends on your Driver aids and the Affinity of the Car. Make sure not to use rewind because it adds up. Don't be afraid to slow down a little to pass the lapped cars. You average lap should be around 43.700
All together you get around 110000Cr in less than 15 minutes!
If you are lazy and need a decent tune, go to my storefront Hir0pr0tagon1st. (Hir(zero)pr(zero)tagon(one)st). I named the setup Indy CR GRINDIN. Its a simple setup, you could probably make something better, but its top speed tuned to 210 mph and I messed with the suspension to decrease tire degradation. You could probably do races upwards of 50 laps without pitting if you really wanted to. I do 20 laps just because i'm paranoid, but if you use your VIP tokens on xp boosts, you want a longer race. I put reddit in the tune description. Even if you don't use it downloading it would be nice and would help me out. Hope this money making method helps you guys.
Forza 5 Credit Grinding Method for those with E21 F1 Car