Ballast to 50/50 weight balance, install race suspension/brakes/anti-roll bars, add aero package, set ride height to minimum. Everything below tunes the car after that's done.
Driving style
Pick how you drive. This reshapes everything below.
Inputs
Forza shows the class letter on the car selection screen and the Upgrade Shop. It updates as you change parts.
Modern = 1995 or newer. Classic / Vintage = 1994 or older. Year is shown on the car selection screen.
Forza shows car weight on the upgrade screen. Heavier cars need higher tire pressures to support the load. Default 3,000 lb if unknown.