The downforce numbers that the game gives you for various cars are extremely contradictory and generally meaningless outside of tuning for that specific car. Some old 60s rally cars have 15/35 despite having nothing in the way of spoilers, even though that's the same downforce rating that the modern WRC cars have. The 1960s Le Mans cars have the same downforce numbers as the JGTC cars, despite in real life not having any downforce at all (and JGTC cars having so much downforce that cars from other contemporary racing series could not compete).
It's really just kind of a mess.