A Hydra-Matic Update

I have spent a lot of time over the past few weeks undertaking an extensive update of the 2010 article on the original GM Hydra-Matic transmission. The goal was to clear up various errors and points of confusion as well as trying to do a better job of explaining the operating principles of both Hydra-Matic and its precursor, the Automatic Safety Transmission.

This endeavor, which has reminded me why I’m neither a mechanic nor an engineer, was actually a good deal more work than the original draft. However, since people continue to read and refer to this article, I felt it was appropriate to try to sort out its inaccuracies and confusing points.

You can read the full article here.

Click below for two additional notes on this update.


Among the points I wanted to clear up in this draft was the correct gear ratios of the original single-coupling Hydra-Matic. There’s a lot of conflicting information on this point, compounded by the fact that until the late forties, Oldsmobile and Cadillac didn’t use the same ratios. (The ratios were standardized around the 1948 model year.) As best I can determine, the correct ratios — discounting rounding variations — are as follows:

Hydra-Matic gear ratios for 1940–1955 single-coupling transmissions: Early Oldsmobile: 1st: 3.66, 2nd: 2.53, 3rd: 1.44, 4th: 1.00, Reverse: 4.31. Early Cadillac/military: 1st: 3.26, 2nd: 2.26, 3rd: 1.44, 4th: 1.00, Reverse: 3.77. Early postwar automotive: 1st: 3.82, 2nd: 2.63, 3rd: 1.45, 4th: 1.00, Reverse: 4.31. Late postwar (1955) automotive: 1st: 4.10, 2nd: 2.63, 3rd: 1.55, 4th: 1.00, Reverse: 4.62. Not all users adopted the late ratio set.

Please note these are internal gear ratios, NOT axle or transfer case ratios, which differed greatly from application to application. Postwar military units combined the 2.53:1 rear ratio from prewar Oldsmobiles with a new 1.55:1 front gearset, giving a first gear ratio of 3.92:1; that combination wasn’t used on passenger cars.


Before anyone asks, I have NOT yet updated the second part of the article, covering Dynaflow, Powerglide, and the second- and third-generation Hydra-Matics. I still hope to do so, but that article is substantially more complicated than this one and will take more time. (It involves eight different transmissions of four substantially different types!)

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