The AI is known to be non-effected by fuel loads, while the player is effected. This is regardless of race lengths. On 100% races this issue is that much more obvious, due to the fact there there are 40 - 60 laps in a 100% race.
The AI starts by doing 3 - 4 seconds faster than you for the first half of the race. By this time you're getting lapped by other cars. By the end of the race the AI will have lapped you about 2 - 3 times.
Simple test:
Turn fuel simulation on. Start a race. Can you keep up with the AI off the line? No, not with fuel simulation on.
Now, turn fuel simulation off. Start a race. Can you keep up with the AI off the line? Yes, with fuel simulation off.
The AI starts by doing 3 - 4 seconds faster than you for the first half of the race. By this time you're getting lapped by other cars. By the end of the race the AI will have lapped you about 2 - 3 times.
Simple test:
Turn fuel simulation on. Start a race. Can you keep up with the AI off the line? No, not with fuel simulation on.
Now, turn fuel simulation off. Start a race. Can you keep up with the AI off the line? Yes, with fuel simulation off.

