VehicleWheelWheelFrictionSlip Property

This determines how much grip this wheel has. It is combined with the friction setting of the surface the wheel is in contact with. 0.0 means no grip, 1.0 is normal grip. For a drift car setup, try setting the grip of the rear wheels slightly lower than the front wheels, or use a lower value to simulate tire wear.

It's best to set this to 1.0 when starting out.