If I remember correctly, the best "credit per minute" comes from the a-spec all american championship event at indy speedway, preferably using a fully tuned Corvette ZR1 (C6) RM '09 (Racing Modified) with minimum rear downforce for the best time.
-Look around the internet, there is a chart somewhere that tells you the exact amount of credits you gain per minute, including loading times. for both a-spec and b-spec.