The grip of the tyres means the frictional force between the tyre and the road. The grip between tyre and road depends on two factors.
1. Tyre conditions
2. Road condtions
3. Weight on the tyre (Weight of the vehicle)
The friction between two surface can be describes as below.
Friction = Frictional coefficient xx Perpendicular reaction force
If the vehicle is travelling along a horizontal surface the reaction force between tyres and road (perpendicular to the road) would be the weight.
F1 = u x mg
But if the vehicle is travelling on an inlcined road, the perpendicular reaction would be just a component of the weight.
If the angle of the inclination is A, then,
F2 = u x mg sin(A)
mg sin(A) < mg
Therefore the grip or frictional force is less when travelling on a inlcined road than on a horizontal surface. Tyres grip better on a horizontal surface.