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?
I was already thinking how can I get the exact rear end ratio, I will do as you suggest below and tell the result here. I will also do another test drive and measure the exact GPS speed in both gears with my phone.
Thanks again for all the help - I really appreciate it!