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This is the Chevy/New Process A-833 four-speed OD, offered as optional equipment on '81-86 (or '87) Chevy 1/2-ton trucks. Internally, it's the same as the Mopar A-833, with Fourth gear being 0.73:1 overdrive. It's essentially the same as the Mopar muscle car-era trans, but with an aluminum main case rather than the iron case. The 3-4 shifter arm on the Overdrive is installed upside-down from a normal four-speed. When the shifter goes into the Third gear gate, the transmission internals go to what is normally Fourth gear-straight through. Then when the shifter goes into the Fourth gear gate, the internals go through what is normally the Third gearset (but with different tooth counts than a regular Third gear cluster) and comes out the tailshaft as 0.73:1.