The Muncie M20 is basically a wide-ratio transmission and was intended for normal street driving use. The M20 units have a 2.52:1 first gear. The M20 is most compatible with GM 3 Series rear end differential gear ratios such as 3.08, 3.31, 3.55, 3.73, etc. They are for economy and street drivability and just cruising.

The Muncie M21 is a close-ratio unit, ideally designed to be matched with lower gear ratios such as 3.73, 4.11, or 4.56 GM 4 Series rear end differential gears. The M21 units have a 2.20:1 first gear. A close-ratio gearbox keeps shift points closer together, which keeps rpms up. The M22 has higher-nickel-content straight-cut gears. It uses much the same gear ratios as the close-ratio M-21, but can take more abuse. An M22 retains less heat due to its straight-cut gears and the lack of thrust that helical-cut gears create. An M-22 holds up really well, the downside is it tends to make noise. It will rattle at low rpms and whine at high rpms.

The Muncie M22 is also a close-ratio unit, ideally designed to be matched with lower gear ratios such as 3.73, 4.11, or 4.56 GM 4 Series rear end differential gears. The M22 units have a 2.20:1 first gear. The M22 is the one and only "Rock Crusher". Some people think all Muncie four-speeds are Rock Crushers, but they're not.