[Bill Hanlon]

Originally Posted by Onestone

Thank you so much for your help!

I already thought the same way - how to determine which gear 2500rpm at 35mph is

So what I already did is measuring the circumfence of my tires -> it's 223cm, which is almost exactly 87,76" - as shown in the shop manual for 7:60-15 tires.

I did today some pictures from the differential and the transmission - because I was hoping to find anything on the housings that tell me the ratios. See the pictures attached.

On the differential housing I found a number called "A25 50" - Google didn't know anything about that.

And on my transmission there is a plate which says "D56 - 46365" - is this a 1956 dual range Hydramatic on my 1955 maybe?

The "A25 50" is actually a casting date for the diff housing. And the zero is not a number, but a mark in the casting that sort of looks like a flat head screw. There is another such mark at the front of the casting date. GM did this on many major cast parts like engine blocks, heads, transmission cases, etc. After 70 years they can be hard to read.

The remaining A255 decodes as ...
A = January, B = February, C = March, etc
25 is the 25th day of the month (if less than the 10th only one digit is used)
5 is the last digit of the year, 1955

Originally Posted by Onestone

And on my transmission there is a plate which says "D56 - 46365" - is this a 1956 dual range Hydramatic on my 1955 maybe?

Likely, but my Pontiac decodes only go thru 1955. The '55 would be a "P55", but remember in 1956 Pontiac used two automatic transmissions, the dual-range HydraMatic like you have and the Strato-Flight Hydra-Matic (also known as Controlled Coupling Hydra-Matic or Super Hydra-Matic). Those would have a different letter/number on the plate.

I've updated my spreadsheet with your tire size. When you get me the rear end ratio I'll load a new one.

[Jack Gifford]

Bill- the only failure I would expect of a hydro's clutches would be failure-to-engage. A failure-to-engage the front clutch would cause 2nd & 4th to act as neutrals. (Ask me how I know...)
Perhaps he should try driving it in 3rd up to a speed where it would try to shift into 4th and see what happens?