The M6 "four-speed electro-hydraulic semi-automatic" transmission

Thanks to Argent, Bill Watson, and Sam Chase. Additions on shifting technique from S. Berliner's web site (URL now invalid).

The M6, sold as Presto-Matic, Fluidmatic, Tip-Toe Shift, Gyro-Matic, and Gyro-Torque, was a two-speed manual transmission with an electric 1:1 ratio unit (loosely called an overdrive unit) attached. It was used on Dodge (1948-1953), DeSoto (1946-1953) and Chrysler (1946-1953) models, and was coupled to either Fluid Drive (fluid coupling) or Fluid-Torque Drive (torque converter).


The driver would select either low or high range (the two manual forward gear sets) and "shift" the transmission within either range at a predetermined speed by lifting his foot off the accelerator pedal, causing the overdrive unit to kick in. The four "speeds" were Low, Low Overdrive, High, and High Overdrive.

Most people would start out in High range and somewhere between 15/20 mph, lift their foot off the accelerator, whereby the OD unit would kick in for "cruising" gear. The OD would kick out at any speed under 11 MPH, and when the car was brought to a stop. For better low-speed acceleration, you could start in the low range and first gear, at 8 mph release the gas and wait for the clunk as it shifted, then go up to 25 and shift into the high gear.

To use all four speeds, you could start in low range, and, while shifting to high range, floor the gas, which caused the transmission to kick down to the low gear, bringing third gear into play. This could provide a performance boost.

The clutch was necessary any time the gear lever was moved between Low/High, or Reverse. A fluid coupling was attached to the flywheel, and a conventional clutch was mounted in tandem.

Later models had shift quadrants to make the configuration appear as "automatic" as possible. On the 1954 Dodge, the quadrant had four positions in this order: R L Nu [Neutral] Dr, with the same H pattern as a conventional 3-speed, minus 1st gear ("L" was in the 2nd gear position, "Dr" in the 3rd gear slot.)

Bill Watson noted "The shifting of gears within low or high range was not done automatically as you had to lift your foot off the accelerator and wait for the "clunk" when the higher gear engaged. Some, including Chrysler, describe it as a 'click,' but I used to own a 1949 DeSoto and it was a 'clunk'! The downshift occurred automatically when the car slowed down to a stop."

Fluid Drive referred to the fluid coupling between the engine and the clutch, and had nothing to do with the transmission. The Fluid-Torque Drive (torque converter) arrived for the 1951 model year as an option. This combination got a shift quadrant, but as it was coupled to the M-6, it still had a clutch.

Mike Sealey noted that the M-6 was marketed under the name "Prestomatic" at Chrysler, "Tip-Toe Shift" at DeSoto, and "Gyro-Matic," "Fluid-matic," "Fluidtorque," and "Gyrotorque" at Dodge. Check out the model name badge on second series 1949 through 1952 Dodge Meadowbrooks and Coronets; if it says "Fluid Drive" below the model name, that originally meant the standard transmission with the fluid flywheel action, while if it says "Gyro-Matic", that emblem belongs on a car with the M-6. (Sam Berliner III wrote: “My 1941 Chryslers all said "FLUID DRIVE" on the trunk light cluster, even my Windsor with a 3-speed manual and the fluid coupling, as did my 1950s — even my 1949 C-46 Highlander 8 does — bracketing the back-up light").

The M-6 transmissions often confuse modern drivers who believe them to be 3-speed sticks that refuse to go into first. Besides the clutch pedal, later versions can be told apart from the later PowerFlite by their strange "D-N-R-L" range of gears - the position of reverse making a strong argument for keeping the clutch pedal!

Bob J. wrote:

Regarding the M-6 transmission in the 1946,47,48 Chrysler and DeSotos - I have a 1948 DeSoto with this transmission that I have disassembled and reassmbled using the 1948 DeSoto Shop Manual. It has 2 gear ratios in low range in the column shift pattern and two gear ratios in the high range of the column shift pattern. It is very difficult to use all four gear ratios in a normal start.

Starting in low range, first gear ratio, and then allowing the automatic shift into the second gear ratio also positions the internal gear ratio into the fourth gear ratio. In effect, the transmission uses only 1st, 2nd and fourth gear ratios. Third gear is skipped because the transmission internally has already made its hydraulic shift into fourth gear ratio. The only way to pick up third gear ratio is to depress the accelerator pedal to the floor, through the detent, causing the transmission to automatically do its hydraulic shift back into third gear ratio.

All this is exactly as the shop manual details the operation and that is exactly as the parts work in the transmission.

Two other points: The foot operated clutch is strictly mechanical and not hydraulic. The fluid coupling is just that - a fluid coupling and does not multiply torque therefore is not a torque converter.

I hope this further clarifies some of the information on this transmission as there seems to be a great deal of confusion as to how it really works If anyone has further comments or questions I would be happy to discuss further.

Hemi Andersen wrote:

Cars that had the name 'Fluid Drive' on them had only a fluid coupling, no special transmission. That was mainly the Dodge. Dodge had Gyro-Matic which was fluid coupling with a M-6 transmission. Gyro-Torque was the self contained torque converter with the M-6 transmission. The distinction between 'Fluid' and 'Torque' told you which hydraulic coupling was used. Chrysler & DeSoto used the M-6 across the board as I recall.

The M-6 like the cast iron Powerflite and Torqueflite transmission were bolt-on to the bellhousing transmissions with various hydraulic couplings ahead of them. In those days the coupling was not part of the transmission. The couplings were directly bolted to the crankshaft, so when you removed the transmission, the coupling was left attached to the engine.

M4 (Vacamatic / Simplimatic)

The M4 was a pre-war 4-speed, vaccuum-hydraulic unit used on Chryslers and DeSotos in 1941 and 1942. Chrysler called it the Vacamatic and DeSoto the Simplimatic, in the U.S. Chrysler of Canada sold the unit as Simplimatic on both DeSoto and Chrysler cars. It was not available on the Dodge. The Vacamatic / M4, as one might expect, was operated by engine vacuum. Like the M6, it had a low and high range, and a low and high gear, providing four speeds.

S. Berliner III wrote: "Don't know if it applied to the M6 or only the M4 Vacamatic but one of my Dad's '41s, the Royal or the Saratoga, had a three-speed Slushomatic, not a four-speed; there was shifted first and shifted second with tip-toed third, only. My own '41 Windsor was Fluid Drive on a 3-speed manual, which I loved. Also, my aunt's 1938 Olds had a THREE-speed Hydramatic and Dad's '50 Chevy had a totally-unsatisfactory TWO-speed Powerglide!"

Other Chrysler, Dodge, Plymouth, DeSoto, and Jeep transmissions.

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