The M22 4 speed or commonly known as the “rockcrusher”, differs from M21, though they had the same gear ratios, with its heavy duty gearset.  This was designed primarily as a road race transmission.  Its straighter angle on the gearset produced less heat and less end loading of the gear train.  In drag race applications, this 4 speed combined with a high impact alloy gears, would really pushed the limits of its aluminum case.  Despite its shortcomings like the gears are not spur gears, they were still able to produce a reasonable amount of gear noise, so it was named “Rockcrusher”.  Some of the movie that features the M22 best was the “Two Lane Black Top” and the “American Graffiti”.

M22 and M21, both have the same dead 2.20 first gear ratio, which is not good for off the line acceleration.  To compensate for it, at least 4.56 rear end gears are needed.  In 1967, it was still manageable because gas then was cheaper, and pulling 3000 RPM at 50 MPH is not an issue, but as time goes on and so does the requirements, M21 and M22 are not practical and wise for today’s driving habits anymore, making them the closest ratio production 4 speeds ever produced. It’s only good for road racing. 

Mustang GT's with a 3.35 1st gear and 3.08 rear, New Camaros with a 2.68 1st gear, 3.73 rear gear and .59 overdrive, ZR1's with a 2.68 1st gear, 3.54 rear gear and a .59 overdrive are examples of today’s new car gear set ups.