I'm just saying that the chassis has a "grip" modifier in it. We learned this from the hackers. I'm of the opinion that having this be a part of the chassis is problematic and quite likely the reason why there are some outstanding imbalances in the game.
I believe they took this approach so they could better replicate a car's factory behaviour, which to a great part is dictated by the tires that appeared on those cars. For instance, the 427 Cobra had bias ply tires on it. The F40 has Pirelli P zero, but from the 90's.
In my opinion, since we are playing the game today, the upgrades should be up to today's standard. So if I fit a SS tire on the Cobra, I would expect it to be the same SS tire that is fitted to everything else.
If they want to replicate original equipment grip levels, they should have an OE tire class and the grip modifier should reside there.
But this is a discussion for some other update in the future.