I Posted this in a specific thread.. but realized many could benefit from it...
It has a plethora of Mazda related interchange information.. there even are KIA crossovers..

http://members.aol.com/solomiata/Drivetrain.html

FOR EXAMPLE....
The 1990-93 rear had 4.30:1 gears and there are no factory alternate ratio's available (an aftermarket 4.875 ratio is available). Mazda OEM Gears currently available new for the later 1994+ differential assembly are 3.636:1, 3.909:1, 4.10:1, 4.30:1, 4.44:1, 4.625, and 4.778. The 4.10:1 and the 4.30:1 came on the 1994-97 and '99+ Miata respectivley (4.10:1 on automatic starting in 2000). The 4.44:1 came as original equipment on 1987-88 4x4 trucks on the front axle and some commercial light duty trucks. The 3.909 came as original equipment on the 79-85 RX-7 12a as well as the 99+ 6-speed Miata. The 3.636:1 came on the 79-82 RWD 626 5-speed and is currently available new on the 2001+ Austrailan 6-speed. The 4.625 and the 4.778 come on the Kia Sportage 4x4 on the front axle. The 4.778 is US spec and the 4.625 is unique to the NZ market. Mazda has discontinued the 3.308 (optional on 82-83 B2000 diesel) gears. Steeper 4.875:1 and 5.125:1 aftermarket gears are available. All gears are interchangable as a set. See also Felix's RX-7 drivetrain interchange page page for older model Mazda ratio info. Ring and pinion tooth counts