Pneumatic tire

1. A pneumatic tire comprising:

at least two main grooves extending in a circumferential direction;

a plurality of tire widthwise directional grooves that intersect the main grooves in a tire tread portion, wherein the tire widthwise directional grooves are terminal grooves branched off from one of the at least two main grooves;

a region on an inner side in a widthwise direction of the tire of tire widthwise directional center lines of outermost main grooves with respect to the widthwise direction of the tire constituting a tread center portion, and regions from the tire widthwise directional center lines of the outermost main grooves with respect to the widthwise direction of the tire to tread end portions at outer ends in the widthwise direction of the tire constituting tread shoulder portions; and

the tread center portion having at least three pitch variations in terms of pitch length in the circumferential direction of the tire in regions formed by the tire widthwise directional grooves and land portions adjacent to the tire widthwise directional grooves to one side in the circumferential direction of the tire, and, taking n as pitch count and labeling respective pitch lengths Pc from longest to shortest as Pc 1 , Pc 2 , Pc 3 , . . . , Pcn, relationships Pc 1 /Pc 2 ≧Pc 2 /Pc 3 ≧, . . . , ≧Pcn−1/Pcn and Pc 1 /Pc 2 >Pcn−1/Pcn holding true;

wherein there are at least three pitch variations in terms of pitch length in the circumferential direction of the tire in the regions formed by tire widthwise directional grooves and the land portions adjacent to the tire widthwise directional grooves to one side in the circumferential direction of the tire in the tread shoulder portions, and, taking n as the pitch count and labeling the respective pitch lengths Ps in order from longest to shortest as Ps 1 , Ps 2 , Ps 3 , . . . , Psn, relationships Ps 1 /Ps 2 ≦Ps 2 /Ps 3 ≦, . . . , ≦Psn−1/Psn and Ps 1 /Ps 2 <Psn−1/Psn holding true.

2. The pneumatic tire according to claim 1 , wherein a minimum pitch length Pcn in the tread center portion and a minimum pitch length Psn in the tread shoulder portions satisfy the relationship Pcn>Psn.

3. The pneumatic tire according to claim 2 , where a total number of pitches having pitch length Pc 1 along a tire circumference in the tread center portion is less than a total number of pitches having pitch length Pcn along the tire circumference.

4. The pneumatic tire according to claim 3 , wherein a total number of pitches having a pitch length Ps 1 along the tire circumference in the tread shoulder portions is less than the pitch count for Psn along the tire circumference.

5. The pneumatic tire according to claim 4 , wherein, taking n as the pitch count, at least one selected from the group consisting of a pitch length ratio Pck−1/Pck in the tread center portion (k being at least one natural number from 2 to n) and a pitch length ratio Psk−1/Psk in the tread shoulder portions (k being at least one natural number from 2 to n) is in a range from 1.05 to 1.20.

6. The pneumatic tire according to claim 5 , wherein at least one selected from the group consisting of a ratio Pc 1 /Pcn of a maximum pitch length Pc 1 to a minimum pitch length Pcn along the tire circumference in the tread center portion and the ratio Ps 1 /Psn of a maximum pitch length Ps 1 to a minimum pitch length Psn along the tire circumference in the tread shoulder portions is in a range from 1.20 to 2.00.

7. The pneumatic tire according to claim 2 , wherein a total number of pitches having pitch length Ps 1 along a tire circumference in the tread shoulder portions is less than a total number of pitches having pitch length Psn along the tire circumference.

8. The pneumatic tire according to claim 7 , wherein, taking n as the pitch count, at least one selected from the group consisting of a pitch length ratio Pck−1/Pck in the tread center portion (k being at least one natural number from 2 to n) and a pitch length ratio Psk−1/Psk in the tread shoulder portions (k being at least one natural number from 2 to n) is in a range from 1.05 to 1.20.

9. The pneumatic tire according to claim 8 , wherein at least one selected from the group consisting of a ratio Pc 1 /Pcn of a maximum pitch length Pc 1 to a minimum pitch length Pcn along the tire circumference in the tread center portion and the ratio Ps 1 /Psn of a maximum pitch length Ps 1 to a minimum pitch length Psn along the tire circumference in the tread shoulder portions is in a range from 1.20 to 2.00.

10. The pneumatic tire according to claim 1 , where a total number of pitches having pitch level Pc 1 along a tire cicumference in the tread center portion is less than a total number of pitches having a pitch length Pcn along the tire circumference.

11. The pneumatic tire according to claim 1 , wherein a total number of pitches having pitch length Ps 1 along a tire circumference in the tread shoulder portions is less than a total number of pitches having pitch length in the tread shoulder portions is less than a total number of pitches having pitch length Psn along the tire circumference.

12. The pneumatic tire according to claim 1 , wherein, taking n as the pitch count, at least one selected from the group consisting of a pitch length ratio Pck−1/Pck in the tread center portion (k being at least one natural number from 2 to n) and a pitch length ratio Psk−1/Psk in the tread shoulder portions (k being at least one natural number from 2 to n) is in a range from 1.05 to 1.20.

13. The pneumatic tire according to claim 1 , wherein at least one selected from the group consisting of a ratio Pc 1 /Pcn of a maximum pitch length Pc 1 to a minimum pitch length Pcn along a tire circumference in the tread center portion and the ratio Ps 1 /Psn of a maximum pitch length Ps 1 to a minimum pitch length Psn along the tire circumference in the tread shoulder portions is in a range from 1.20 to 2.00.

14. The pneumatic tire according to claim 1 , wherein the at least two main grooves are linear grooves.

15. The pneumatic tire according to claim 1 , wherein the tire widthwise directional grooves in the tread center portion are oblique to the tire circunferential direction.

16. The pneumatic tire according to claim 1 , wherein the tire widthwise directional grooves in the tread center portion are formed in a circumferential rib.