55.2-59 1957 Chevy Hydramatic rear end gear change

[1project2many]

I have owned two Hydramatic trucks although I only got to drive one. They are different from what we're used to in a few ways. The Hydramatic transmission has no torque converter. Instead it uses a simple fluid coupling with no torque multiplication. This results in more slip when the truck is under load and when starting from a standstill. There is not park position so the park brake should be kept operational. If "in gear" parking is desired the transmission should be shifted to reverse then the engine can be shut off once reverse is engaged. There is no ability to select a specific gear other than first. The shifter is designed to limit upshifts. A show truck will not have troubles with this but for some this was problematic in work trucks. A fully loaded truck could also show problems with unexpected upshifts. Sufficient pressure could be generated so a vehicle in a lower gear range might accidentally shift to the next gear causing a substantial reduction in engine braking. Again, a show truck is not likely to have a problem with this.

Was the donor truck a 1/2 ton or was it a larger truck? 1/2 ton transmissions came with a different 1st and 2nd gear ratio.

[markymark77]

Originally Posted by 1project2many

I have owned two Hydramatic trucks although I only got to drive one. They are different from what we're used to in a few ways. The Hydramatic transmission has no torque converter. Instead it uses a simple fluid coupling with no torque multiplication. This results in more slip when the truck is under load and when starting from a standstill. There is not park position so the park brake should be kept operational. If "in gear" parking is desired the transmission should be shifted to reverse then the engine can be shut off once reverse is engaged. There is no ability to select a specific gear other than first. The shifter is designed to limit upshifts. A show truck will not have troubles with this but for some this was problematic in work trucks. A fully loaded truck could also show problems with unexpected upshifts. Sufficient pressure could be generated so a vehicle in a lower gear range might accidentally shift to the next gear causing a substantial reduction in engine braking. Again, a show truck is not likely to have a problem with this. Was the donor truck a 1/2 ton or was it a larger truck? 1/2 ton transmissions came with a different 1st and 2nd gear ratio.

Thank you so much for the description. The truck would just be for driving around town and will not be a show truck. It wont be used to haul anything substantial. The whole drive train came from a 1/2 Chevy truck. I was told that the rearend gears were different (higher), but from documents there were only the 3:90 and 4:11 option. I have a 3:90 and was thinking of changing it to a 3:38, would that be wise with this transmission? Do you anticipate any issues?

Thx!