I flipped back and forth about this, but I figured it out a while back. It's not an emulation problem, just something odd about the game, even on arcade. There's an option in the settings for Special Car. I don't know why they did this, it doesn't make any sense and kinda cons you out of money. In every game where there's an advertised top speed on the car select screen, you can more or less reach it if you're racing well. In Le Mans 24, you can only do that with one standard car at a time, the one car you define as the special car.
I noticed it when watching arcade players. You know the lineup of cars? If a special car is defined, this one will be highlighted first when you press start. If you pick anything else, you basically can't win because you're much slower. The secret cars remain unaffected.