suzuka in x1 is the fastest way to grind exp points. After you reach level 35 it's better do take a day off and go to delasathre because during 19 hours of driving out of 24 you will get 4.500.000 exp points so from his point there is no sense going back to suzuka race.
Calculation is easy
[lvl 40] 7.576,732 +
[lvl 39 ] 5.919,322 +
[lvl 38] 4.624,470 +
[lvl 37] 3.612,867 +
[lvl 36] 2.822,552 = 24.555,943 thats
twenty four million. Five-hundred fifty five thousand, nine-hundred forty three exp points from level 35 do 40 !!!
You take 24.555,943 divide by delasathre amount / 4.593,700xp and you have your number which is 5,34 times. That means fife times you would have to drive delasathre 24 hour race to go from 35 to 40 level. Is this number is scary

Yes but not in a month time period!!! During 30 days five of them you would have to sacrifice just for grinding. And thats on a-spec because in b-spec this number is twice much higher. That's because in b-spec you have the same amount of epx required for go to each level but the overall number for race is half. When in a spec you have 4.593,700xp for la sathre race, but in b spec it's just 2.200.000, well a little more then 2.200.000 but still it's half of what you get for a spec.
So in b spec mode from 35 to 40 level you would have to let bob 10 times, thats ten days