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The 420 & 465 are truck-type transmissions with extra-high-ratio low gears (over 6.5:1) that are used for getting extremely heavy loads moving from a dead stop up to 10mph or so. Without a load, you'd normally start off in 2nd gear, which feels like low gear in a regular 3-speed trans does.

The M-20, M-21, & M-22 are car-type 4-speeds. M-20 has a 2.52:1 low gear, and the others have 2.20:1. The M-22 is a heavy-duty version with gears that are cut at a smaller angle. The smaller angle causes more noise, thus the name "rock-crusher".

Fourth gear in all of them is 1:1.
 
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