Saginaw Transmission Identification

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The Saginaw three- and four-speed manual transmissions transmitted engine power to the wheels on General Motors non-performance and mid-range performance cars. The transmissions were cast-iron and stamped with an application code.

Physical Characteristics

  • Both versions of the Saginaw transmission featured the case and extension housing sealed with a seven-bolt cover on the side. The gears were fully synchronized and the synchro rings had 30 teeth. Mounted on the cover was the reverse shift lever.

Application Codes

  • Saginaw transmissions featured application codes on the passenger side of the case near the rear cover. "O" denoted the transmission was equipped with overdrive. "S" identified the three-speed transmission as manufactured in Saginaw, Michigan, from 1962 to 1979. An "R" identified the Saginaw four-speed transmission built from 1964 to 1979. General Motors used the in-house Regular Production Option (RPO) code "M15" to identify the three-speed and "M20" for the four-speed.

Gear Ratios

  • Saginaw three-speed transmissions featured gear ratios of 2.85:1 for first, 1.68:1 for second and 1.00:1 for third when matched with L6 in-line six-cylinder engines. For small-block V-8s, like the 350, the three-speed used 2.54:1, 1.50:1 and 1.00:1. The four-speed matched with the straight-sixes were 3.11:1, 2.20:1, 1.47-to-1 and 1.00:1. For V-8s, it was 2.54:1, 1.80:1, 1.44:1 and 1.00:1.

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