Part 1a
First have a car that is a class D-A or either class bike. Next, when you start the Tour of Island race, go to the part in the beginning when it wants you to go on the wrong side of the road onto the freeway. Go straight, then follow the way there (you will be on the right side of the road).

Part 1b
Near the part that you go to the left and drive around that fence, go straight and it will be fast (stay on the side that goes the right way).

Part 2
Near the end, when you are on the freeway, continue going straight and when it tells you to go to the left. Then finish the race and you will win (99.9% of the time if you do it right)

Note: I have done this cheat in a class G vehicle and I won. It wasn't the Shelby Daytona Coupe, it was a Lamborghini!