Roto Hydramatic was an automatic transmission

Automatic transmission

An automatic transmission is one type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually...

 built by General Motors and used on Buick and on some Oldsmobile

Oldsmobile

Oldsmobile was a brand of American automobile produced for most of its existence by General Motors. It was founded by Ransom E. Olds in 1897. In its 107-year history, it produced 35.2 million cars, including at least 14 million built at its Lansing, Michigan factory...

 and Pontiac

Pontiac

Pontiac was an automobile brand that was established in 1926 as a companion make for General Motors' Oakland. Quickly overtaking its parent in popularity, it supplanted the Oakland brand entirely by 1933 and, for most of its life, became a companion make for Chevrolet. Pontiac was sold in the...

 models from 1961-1964. It was based on the earlier, four-speed Hydramatic

Hydramatic

Hydramatic was an automatic transmission developed by both General Motors' Cadillac and Oldsmobile divisions. Introduced in 1939 for the 1940 model year vehicles, the Hydramatic was the first fully automatic mass-produced transmission developed for passenger automobile use.-History:During the...

, but was more compact, providing only three forward speeds plus a small 8" fluid coupling with a stator in place of the Hydramatic's fluid coupling

Fluid coupling

A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch...

. Oldsmobile, one of the users of this transmission, called the torque converter's stator the "Accel-A-Rotor." The lightweight, aluminum-cased transmission was sometimes nicknamed the "Slim Jim."

There were two models of the Roto Hydramatic: the lightweight Model 5, which weighed 145 lb (65.8 kg) and had ratios of 3.03, 1.58, and 1.00, and the larger Model 10, which weighed 154 lb (69.9 kg) and had ratios of 2.93, 1.56, and 1.00.