You can configure a reduced wheel radius and tweak the tire friction by calling these methods of the vehicle controller component:
vehicle.SetWheelRadius (wheelIndex, originalRadius - 0.15f);
vehicle.SetWheelTireFrictionMultiplier (wheelIndex, 0.2f);
wheelIndex is the index of the wheel. Wheels are indexed left to right, then front to rear. In standard 4-wheeled cars front wheels are 0, 1 (left, right) and rear wheels are 2, 3.