Set Friction Coefficients

Simulate -> Full Vehicle Assembly -> Driveline Tests -> Torque Tip IN - Tip Out -> Friction coefficient
If you have an assembly with rtire models then you can modify the friction characteristics for each tire.
 
For the option:
Do the following:
Wheel Subsystem
Select the wheel subsystem from the option.
Define Friction for
Left Tire
Right Tire
Time Based /Distance Based
Initial Friction
Enter the value for initial friction.
Intermediate Friction
Enter the value for intermediate friction.
Final Friction
Enter the value for final friction.
Time 1 / Distance 1
Enter the value for time 1 / distance 1.
Delta 1
Enter the value for delta 1.
Time 2 / Distance 2
Enter the value for time 2 / distance 2.
Delta 2
Enter the value for delta 2.
Define Explicit Function
Select to define function expression.
Function Expression
Enter the value for function expression.
Set parameters
Select to set the parameters.