1950 Dodge Coronet D34 Axle Ratio

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All the literature and classic car databases I have seen concerning drive lines for the 1949-1952 Dodge Coronet (mine is a 1950 D34) say that the axle ratio on the 3-speed manual models is 3.9.  That is incorrect.


All 1949 Dodges came standard equipment with Fluid Drive, which is NOT the transmission; it is a hydraulic (fluid) coupling that takes the place of the conventional flywheel.  With the fluid drive, you had a choice of two transmissions:  a conventional 3-speed manual that did not shift by itself, or a Gyro-Matic transmission, a so-called semi-automatic which was in reality a 4-speed manual unit that had two forward speeds, a low 2.14-to-1 gear, and direct drive 1 to 1.  It also had a 1.75 reduction gear that would engage and disengage automatically with either the low gear or direct drive, depending on the position of the gear lever.


According to my D34 Coronet 1950 Owner's manual, the D34 with the standard three speed came with a 4.1 axle, and if it had the Gyromatic, it came with the 3.9.  My shop manual confirms this, as it lists the 4.1 axle as standard for the 3-speed D34, with optional ratios 3.73 and 4.30.  It does not list the 3.90 as being available with the 3-speed.  The shop manual shows that Gyromatic came with a 3.9 on the Sedan, and a 4.1 on the wagon.

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Is this a question?

The ratio is how many times the driveline rotates in relation to how many times the wheels rotate.

Jack up the car and make a reference mark on a wheel and one on the driveline.

Rotate the wheels being sure that they turn together while counting the driveline revolutions.

3 3/4 turns + 3.73. 4.3 turns = 4.30, Just under 4 turns is probably 3.90 and etc. (if the ratios are consistent thru the years it should actually be a 3.91-1 ratio (common gear in Mopars)).

Pretty easy, just make sure that you rotate the rear wheels together in the same direction.

Sometimes you will even find a tag under one of the third member housing nuts with the ratio stamped on it.

Or take the third member out and count the teeth and do the math.

Fun stuff !!

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My 1935-53 Parts manual shows 4 different rear axle differentials for Dodge.  3.9 to 1 for 1937-1952 with a foot note.  But it also shows 3.73 to 1 for 1949-52 (D29 3 cp., Roadster) and 4.1 to 1 for 49-52 Dodge.  3.9 to 1 D47 for 1953.    You probably need to do Jacks test above to verify what you have.


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