Pneumatic tire

1. A pneumatic tire comprising:

at least three circumferential grooves extending in a tire circumferential direction in a tread portion; and

at least four ribs defined by the at least three circumferential grooves, the at least four ribs extending in the tire circumferential direction; wherein

at least two of the ribs adjacent to each other and including the rib disposed on a first side of a tire equatorial plane in a tire width direction and the rib disposed on a second side protrude to an outer side in a tire radial direction beyond a profile line of a tread surface when viewed in a meridian cross section;

the protruding amount decreases in order from the first side in the tire width direction to the second side;

a groove surface area ratio of the tread surface is greater on the second side of the tire equatorial plane in the tire width direction than on the first side; and

the ribs protruding beyond the profile line are provided adjacent to one another interposed by one of the circumferential grooves, and a difference between the protruding amounts of the adjacent ribs is from 0.4 mm to 0.8 mm, both inclusive.

2. The pneumatic tire according to claim 1 , wherein the protruding amount beyond the profile line of the ribs is from 0.05 mm to 2.0 mm, both inclusive.

3. The pneumatic tire according to claim 1 , wherein the groove surface area ratio Ra on the side on which the rib has the larger protruding amount is from 0.20 to 0.30, both inclusive; the groove surface area ratio Rb on the side on which the rib has the smaller protruding amount is from 0.25 to 0.40, both inclusive; and the groove surface area ratios Ra, Rb satisfy the relationship Ra<Rb.

4. The pneumatic tire according to claim 1 , wherein a vehicle inner/outer side orientation when mounted to a vehicle is designated, the protruding amount of the ribs decreases in order from the vehicle outer side to the vehicle inner side, and the groove surface area ratio of the tread surface is greater on the vehicle inner side than the vehicle outer side.

5. The pneumatic tire according to claim 1 , wherein the ribs protruding beyond the profile line are each provided between the circumferential grooves.

6. The pneumatic tire according to claim 2 , wherein the groove surface area ratio Ra on the side on which the rib has the larger protruding amount is from 0.20 to 0.30, both inclusive; the groove surface area ratio Rb on the side on which the rib has the smaller protruding amount is from 0.25 to 0.40, both inclusive; and the groove surface area ratios Ra, Rb satisfy the relationship Ra<Rb.

7. The pneumatic tire according to claim 6 , wherein a vehicle inner/outer side orientation when mounted to a vehicle is designated, the protruding amount of the ribs decreases in order from the vehicle outer side to the vehicle inner side, and the groove surface area ratio of the tread surface is greater on the vehicle inner side than the vehicle outer side.

8. The pneumatic tire according to claim 1 , wherein the ribs protruding beyond the profile line are each provided between the circumferential grooves.

9. The pneumatic tire according to claim 1 , wherein three of the adjacent ribs protrude to the outer side in the tire radial direction beyond the profile line of the tread surface and the protruding amount of the three adjacent ribs decreases in order from the first side in the tire width direction to the second side.

10. The pneumatic tire according to claim 1 , wherein all of the ribs protrude to the outer side in the tire radial direction beyond the profile line of the tread surface and the protruding amount of the all ribs decreases in order from the first side in the tire width direction to the second side.

11. The pneumatic tire according to claim 1 , wherein one of the circumferential grooves is disposed on the tire equatorial plane.