GM 3987921 Red 21 Tooth Speedometer Gear
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Transmission speedo gear. Correct red color and 21 tooth design. Exact fit for all automatic powerglide , turbo 350 and saginaw or muncie manual transmissions. Will not work for automatic turbo 400 transmissions. Factory match for 3.31 rear axle ratio with o/e size tires. Replaces original GM part number 3987921.
To check your Drive Gear designation/size to determine if you have a 1.84 or a 1.76 gear, from outside looking through the hole/opening, use a cardboard gage and measure the width. The 1.84 is 0.610" wide and the 1.76 is 0.531" wide and both have a 8 tooth count.