A common transmission in the 1967 Chevrolet Chevelle was the Muncie four-speed. Chevy enthusiasts today often swap the two-speed Powerglide, the three-speed Turbo Hydra-Matic automatics or the less reliable Saginaw manual for a Muncie.
According to Hemmings, Muncies used on Chevys from 1966 to 1974 with first-gear ratios of 2.52-to-1 or 2.20-to-1 are the best choices. These transmissions adapt well to the small- and big-block V8 engines. The M20 was a wide ratio four-speed; the M21 had close ratio gears.
The 1966 to 1970 M20 wide ratio version best suits the Chevelle. It has a 2.52-to-1 first gear, 1.91-to-1 second, 1.48-to-1 third and 1-to-1 direct fourth. The wide ratio worked best on Chevelles with a 3.55 rear axle ratio.
Like the M20, the Muncie M21 had a standardized gear ratio to match all engine choices in 1967 Chevelles. The M21's gear ratios were 2.20-to-1, 1.64-to-1, 1.28-to-1 and 1-to-1. The M22 "Rock Crusher," so-named for its angled gears and the tendency of inexperienced drivers of close ratio transmissions to grind gears when shifting, featured different angled gear teeth than the M21 to match the 396 and 454 V8s. The M22 matches best with a 3.73 "Positraction" rear axle ratio.
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