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Fluid Drive is the trademarked name that Chrysler Corporation assigned to a transmission driveline combination offered from 1939 through 1953 in Chryslers and Desotos, and from 1939 through 1954 in Dodge models. more...

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The fluid drive element was a hydraulic coupling inserted in place of the flywheel, and performed the same function as a modern torque converter, only without torque multiplication. "Fluid Drive" was the name Chrysler engineers assigned to Chryslers, DeSotos, and Dodges that had conventional clutches and three-speed manual transmissions installed behind the fluid coupling. Many automobile historians confuse Chrysler’s Fluid Drive with the Corporation’s so-called semi-automatic M6 transmission, which was marketed under various names as “Simplimatic” (Chrysler), “Tip-Toe Shift” (DeSoto), and “Gyro-Matic” (Dodge). Unfortunately, Chrysler itself contributed to the confusion by referring to both the standard-shift fluid drive and M6 installations indiscriminately as "Fluid Drive" in much of their marketing and sales literature.

The standard Fluid drive configuration consisted of the fluid coupling, a manual clutch in tandem, and a conventional three-speed manual transmission. The transmission was not automatic in any way; the driver shifted manually through the gears 1st-2nd-3rd, and had to depress the clutch pedal every time he shifted, just as in a conventional “dry clutch” car. The fluid drive system allowed the driver to stop at a light or in traffic and remain in gear without depressing the clutch. The driver could, if he was not concerned about fast acceleration, drive the car all day long in third gear, stopping and starting, without ever having to touch the clutch pedal or gearshift lever unless he wanted faster acceleration, or backing up was required. A driver could leave the car in second gear in heavy city traffic, stoping and starting without having constantly to pump the clutch pedal, and this was the main reason Desotos and Dodges were heavily favored by city cab companies from the mid Forties to early Fifties. A Fluid drive Dodge was far cheaper than a Hydramatic-equipped Pontiac, and had the effect of making city taxi-driving far easier.

The fluid drive fluid coupling was also used in conjunction with Chrysler’s M6 semi-automatic transmissions. The M6 was in reality a two-speed manual transmission with a conventional clutch mounted behind the same fluid coupling unit that was installed in straight Fluid Drive cars. Attached to the transmission was an “underdrive” with a reduction gear of 1.75/1. The shift lever was column-mounted and had three positions: Low (in the “2nd” position of a conventional 3-speed manual unit), High (in the “3rd” position), and Reverse (same as the 3-speed). The clutch had to be depressed every time the gear shift lever was moved. When the lever was put in Low, the car started in “underdrive” low; when the vehicle reached a minimum speed of 6 MPH, the driver lifted his foot off the accelerator, the underdrive unit would kick out and the car would be in Low. Similarly, with the lever in High position, the car would start in underdrive high, and at any speed above 13 MPH, the driver would lift his foot and the car would “shift” into direct drive. This configuration had the effect of providing 4 gear ratios: Underdrive Low, 3.57/1, Low 2.04/1, Underdrive High, 1.75/1, High, 1/1. In order for the unit to work without gear clashing, it contained a freewheeling device (in Underdrive Low and High), and the Owner’s manual cautioned drivers not to use “1st or 3rd” gear when descending hills, because there was no engine compression braking in those free-wheeling ranges. Generally, most drivers started an M6 car in High and accomplished the shift to direct drive somewhere between 13 and 25 MPH by releasing the accelerator pedal and waiting for the “clunk” that signaled the disengagement of the underdrive. An M6 car would automatically shift from High down to underdrive high when car speed dropped below approximately 11 MPH. The M4 Vacamatic had two speeds in Reverse. There was a manual Pull-Cable to lock out the underdrive in the early models. From 1949-1952, Dodge models with the conventional 3-speed Fluid Drive carried front fender emblems that said “Fluid Drive.” The M6 Models had emblems that proudly proclaimed “Gyromatic.”

Read more at Wikipedia.org