Forza Horizon 3: Blizzard Mountain

This is legitimately a great expansion until the very end. It executes its theme well, the courses are fun, and it feels very different from the main game. Unfortunately, the final challenge is absolutely horrible.
Almost the entire game is about using your own car, with parts you yourself chose, to complete every race. My 1975 Ford Bronco won every single Blizzard Mountain race until the last one, and I was ready to use the car that brought me to win the championship.
Except that's not possible, because they give you one specific car to use. It's a gaudy piece of shit, with an exhaust that explodes every time you shift gears, which is incredibly annoying, especially considering the race is half an hour long and full of hairpin turns.
Either way, I begrudgingly used this car that I hated for half an hour, in first place the entire time, not having seen another racer since the very beginning of the race. Eventually, I got to the final straightaway. I held down the trigger, going wide open the entire time, max speed. All of a sudden, here comes Speedy fucking Gonzales up from behind me and I lose. I hadn't seen him since the start line, but here he fucking is right now, faster than I could ever hope to go.
If I had been driving a car I'd chosen, I would have been annoyed, but I'd understand. But the game forced me to drive a car I absolutely hated, which apparently is the slowest thing on the track.
Why?