Fourth Gear

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Fig 1: Identifying Fourth Gear Planetary Operation

Courtesy of MAZDA MOTORS CORP.

GEAR ROTATION SPEED REFERENCE

Planetary gear	Front
Internal gear	N_R (output)
Planetary carrier	N_C (input)
Sun gear	0 (fix)

Suppose the gear ratio in fourth gear is i₄ ,

Fig 2: Identifying 4th Gear Ratio Formula (1 Of 2)
Courtesy of MAZDA MOTORS CORP.
From the result of N_S =0 in formula (2), the relation between the gear ratio in fourth gear and the rotation speed of the front planetary gear set is indicated in the formula below.
(Z_RF +Z_SF ) N_C =Z_RF N_R
Therefore,

Fig 3: Identifying 4th Gear Ratio Formula (2 Of 2)
Courtesy of MAZDA MOTORS CORP.
As a result, the gear ratio in fourth gear is 0.694.