Are the Mazda RX-8 differential ring and pinion interchangeable with the Mazda RX-7 FD3S ring and pinion?

Yes, the Mazda RX-8 differential ring and pinion gears are directly interchangeable with the Mazda RX-7 FD3S. The FD and the RX-8 share compatible rear ends, making this a straightforward swap for FD owners looking to change their final drive ratio ¹.

A common modification is taking the RX-8's 4.44 or 5.12 ratio ring and pinion gears and installing them into the original FD differential housing ³.

When doing this, it is highly recommended to only swap the gears and retain your FD's factory Torsen limited-slip differential (LSD). The RX-8 uses a Tochigi Fuji-style differential, and the FD's Torsen unit is generally preferred ³.

Difficulty: Hard While the parts themselves are interchangeable, installing a new ring and pinion is a precision task. You must know how to properly set the backlash, adjust the pinion depth, and check the gear tooth patterns ³. If you are not experienced with setting up differentials and lack the necessary measurement tools, it is recommended to have a shop handle the final assembly and gear setup to prevent severe driveline damage or gear whine.