Are there any new tuning options (boost pressure, per-wheel suspension etc)?
In this E3 interview Ralph Fulton states there are no new tuning options (other than the visual customization upgrades) in FH3. The list of FM6 options is below. Players can change the tire pressure for all cars from stock setup. Some cars have parts that can be tuned as stock (such as race cars) while most production cars require Race part upgrades be installed to allow changes to tuning settings for that part.
Tires
All cars can change tire pressures without installing any parts. Adjust pressure to create the right contact patch and affect tirewall flex; pressure increases with heat.
Gearing (Transmission: Sport, Race, or Rally)
Sport transmissions allow for setting the Final Drive ratio while Race transmissions allow for setting each gear individually. Lower numbers allow for higher speed, higher numbers provide more acceleration. Watch the benchmark data for 0-60 and Top speed for these effects.
Alignment (Springs: Race or Rally)
Set Front/Rear Camber, Front/Rear Toe, and Front Caster. These affect turning and stability.
Anti-Roll Bars (ARBs: Race)
Set Front and Rear separately. Lower numbers allow more body roll, higher numbers are stiffer.
Springs (Springs: Race or Rally)
Set Front and Rear stiffness, and ride height.
Damping (Springs: Race or Rally)
Set Front/Rear Rebound and Bump stiffness.
Aero (Aero: Forza Aero parts and some Race wings)
Set Front/Rear downforce. Downforce creates more grip and improves steering but more downforce reduces top speed.
Brake (Brakes: Race)
Set brake balance and pressure. These are used to control when wheels lock up, and how much weight transfer loads each end of the car.
Differential (Differential: Sport or Race)
Adjust the wheel lock of the drive axle. This can improve acceleration and cornering. Converting to AWD provides adjustable differential for front, rear, and split.
Wheel Offset
There is no ability to change wheel offset, though different rim choices may have an impact on appearance.
(Written by Manteomax)