Heavy duty pneumatic tire

1. A heavy duty pneumatic tire comprising:

a tread portion provided with a plurality of circumferentially extending main grooves so that the tread portion consists of nine land portions separated by the main grooves;

the nine land portions comprising a shoulder land portion having a tread edge, a first middle land portion adjacent to the shoulder land portion, a second middle land portion adjacent to the first middle land portion, and a third middle land portion adjacent to the second middle land portion;

the shoulder land portion being formed into a circumferentially continuous rib;

the first middle land portion being formed into a row of first middle blocks separated by a plurality of first middle lateral grooves;

the second middle land portion being formed into a row of second middle blocks separated by a plurality of second middle lateral grooves;

each first middle block having a circumferential length greater than that of each second middle block;

the third land portion being provided with a plurality of third middle lateral grooves to form a plurality of third middle blocks, and

widths W 6 of the plurality of second middle lateral grooves being greater than widths W 5 and W 7 of the plurality of first middle lateral grooves and the plurality of third middle lateral grooves, respectively.

2. The heavy duty pneumatic tire according to claim 1 , wherein a tread width is equal to or more than 300 mm, and an aspect ratio is equal to or less than 55%.

3. The heavy duty pneumatic tire according to claim 1 , wherein the nine land portions further comprise a central land portion adjacent to the third middle land portion, and the central land portion is disposed on a tire equator.

4. The heavy duty pneumatic tire according to claim 3 , wherein

the central land portion is provided with a plurality of central lateral grooves to form a plurality of central blocks,

each central lateral groove extends in parallel with an axial direction of the tire, and

each of the first middle lateral grooves, second middle lateral grooves and third middle lateral grooves inclines at an angle with respect to the axial direction of the tire.

5. The heavy duty pneumatic tire according to claim 4 , wherein

the plurality of first middle lateral grooves and the plurality of third middle lateral grooves incline in the same direction, and

the plurality of second middle lateral grooves incline in an opposite direction to the plurality of first middle lateral grooves and the plurality of third middle lateral grooves.

6. The heavy duty pneumatic tire according to claim 4 , wherein the respective central blocks, first middle blocks, second middle blocks and third middle blocks have hexagonal ground contact surfaces comprising a pair of corners that protrude axially both sides, and each ground contact surface is provided with a zigzag sipe connecting the pair of corners.

7. The heavy duty pneumatic tire according to claim 1 , wherein circumferential lengths L 1 of the plurality of first middle blocks are in a range of from 1.03 to 1.12 of circumferential lengths L 2 of the plurality of second middle blocks.

8. The heavy duty pneumatic tire according to claim 1 , wherein the widths W 6 of the plurality of second middle lateral grooves are in a range of from 1.25 to 1.40 times of the widths W 5 of the plurality of first middle lateral grooves.

9. The heavy duty pneumatic tire according to claim 8 , wherein the widths W 7 of the plurality of third middle lateral grooves are greater than the widths W 5 of the plurality of first middle lateral grooves.

10. The heavy duty pneumatic tire according to claim 9 , wherein the widths W 7 of the plurality of third middle lateral grooves are in a range of from 1.03 to 1.08 times the widths W 5 of the plurality of first middle lateral grooves.

11. The heavy duty pneumatic tire according to claim 1 , wherein the widths W 7 of the plurality of third middle lateral grooves are greater than the widths W 5 of the plurality of first middle lateral grooves.

12. The heavy duty pneumatic tire according to claim 11 , wherein the widths W 7 of the plurality of third middle lateral grooves are in a range of from 1.03 to 1.08 times the widths W 5 of the plurality of first middle lateral grooves.

13. The heavy duty pneumatic tire according to claim 1 , wherein

the nine land portions further comprise a central land portion adjacent to the third middle land portion,

the central land portion is provided with a plurality of central lateral grooves to form a plurality of central blocks, and

circumferential lengths L 4 of the plurality of central blocks are equal to or more than circumferential lengths L 3 of the plurality of third middle blocks.

14. The heavy duty pneumatic tire according to claim 1 , wherein

each of the first middle blocks and the second middle blocks are provided with a zigzag sipe,

the zigzag sipe includes a zigzag opening shape in a plane parallel to a ground contact surface of the block and oscillating in a longitudinal direction of the sipe toward a bottom of the zigzag sipe so as to include at least two turning portions while keeping the zigzag opening shape, and

an amplitude L 6 of the zigzag sipe in the longitudinal direction of the sipe is in a range of from 0.5 to 4.0 mm.

15. A heavy duty pneumatic tire comprising:

a tread portion provided with a plurality of circumferentially extending main grooves so that the tread portion consists of nine land portions separated by the main grooves;

the nine land portions comprising a shoulder land portion having a tread edge, a first middle land portion adjacent to the shoulder land portion, a second middle land portion adjacent to the first middle land portion, and a third middle land portion adjacent to the second middle land portion;

the shoulder land portion being formed into a circumferentially continuous rib;

the first middle land portion being formed into a row of first middle blocks separated by a plurality of first middle lateral grooves;

the second middle land portion being formed into a row of second middle blocks separated by a plurality of second middle lateral grooves;

each first middle block having a circumferential length greater than that of each second middle block;

the third land portion being provided with a plurality of third middle lateral grooves to form a plurality of third middle blocks, and

widths W 7 of the plurality of third middle lateral grooves being greater than widths W 5 of the plurality of first middle lateral grooves.

16. A heavy duty pneumatic tire comprising:

a tread portion provided with a plurality of circumferentially extending main grooves so that the tread portion consists of nine land portions separated by the main grooves;

the nine land portions comprising a shoulder land portion having a tread edge, a first middle land portion adjacent to the shoulder land portion, a second middle land portion adjacent to the first middle land portion, a third middle land portion adjacent to the second middle land portion, and a central land portion adjacent to the third middle land portion;

the shoulder land portion being formed into a circumferentially continuous rib;

the first middle land portion being formed into a row of first middle blocks separated by a plurality of first middle lateral grooves;

the second middle land portion being formed into a row of second middle blocks separated by a plurality of second middle lateral grooves;

each first middle block having a circumferential length greater than that of each second middle block;

the third land portion being provided with a plurality of third middle lateral grooves to form a plurality of third middle blocks;

the central land portion being provided with a plurality of central lateral grooves to form a plurality of central blocks, and

circumferential lengths L 4 of the plurality of central blocks being equal to or more than circumferential lengths L 3 of the plurality of third middle blocks.