This is the best paying race I have been able to find.
Supercar seasonal > Eifel race.
Car to use: Mazda Furai (prize car from the B-spec Miata race)
MAIN SETUP:
Ride height: -2 / -4
Spring rate: 16.2 / 17.1
Damp (ext): 7 / 7
Damp (comp): 6 / 6
AR bars: 5 / 5
Camber: 2.2 / 1.8
Toe: -0.10 / 0.18
LSD: 16 / 32 / 18
Downforce: 15 / 30
Brakes: 7 / 6
Ballast: 200kg at -5
Then make the following adjustments depending on whether or not certain cars are in the starting lineup:
If the McLaren F1 and/or Nissan R390 are in the race:
Stage 3 turbo
Transmission: 180mph
Power limiter: 66.3% (or whatever gets you to 560pp)
560pp - 7:35 total race time (laps @ 1:28.xxx) = 3,097,344 credits
If the Tommykaira ZZII, Zonda C12S, and/or VW Nardo are in the race:
NO turbo
Transmission: 174mph
Power limiter: 53.2% (or whatever gets you to 520pp)
520pp - 7:45 total race time (laps @ 1:31.xxx) = 4,185,600 credits
If none of those cars above are there: (you can beat the XJ220 and Zonda C12 with this)
NO turbo
Transmission: 168mph
Power limiter: 50.0% (mine goes down to 512pp, you might still win with a lower pp but you'd have to run dirty oil to lower your power)
512pp - 7:50 total race time (laps @ 1:32.xxx) = 4,408,832 credits
This race has the highest payout per time that I have been able to find. The payout amounts shown here are with the 2x login bonus as well as the 2x "holiday" bonus which ends tomorrow. So after tomorrow, all payouts will be half off what you see here. I know that this is showing credits, not exp amounts, but since the races have a similar credit/exp ratio that doesn't really matter.