The winner of the final race was
H, who placed 3rd or 4th in the practice race.
Explanation:
You have 8 possible places in each race. We know that D was 1st in the practice, 4th in the final, so we can fill him in.
P F
1 D
2
3
4 D
5
6
7
8
Next we know G got worse by 5 spaces, which means he was either 1,2, or 3 in the practice race and 6,7, or 8 in the final race. However, D was 1 in practice, and C was worse than G in the final, so G must have gotten 2 and 7 in the respective races.
P F
1 D
2 G
3
4 D
5
6
7 G
8 C
E and F switched spots in the final race, so they can't be in spots 4,7, or 8 in the practice race. C did better than E and F in the practice race, so they must be in slots 5 and 6 in both races (doesn't matter which order).
P F
1 D
2 G
3
4 D
5 E F
6 F E
7 G
8 C
A and B moved up five spaces in the final race, so they must be in slots 7 and 8 in the practice race and 2 and 3 in the final race (doesn't matter which order).
P F
1 D
2 G A
3 B
4 D
5 E F
6 F E
7 A G
8 B C
That leaves H in the remaining places: 3rd or 4th in the practice race, 1st in the final race.