Speedo calibration and tranny shift points...

Curious if the electronic speedo gear sensor is hooked up to the auto transmission in any way? I am assuming the computer decides when to up/down shift based on throttle position and speed, am I wrong? So if say you throw 33"s and 4.56's in; the tranny will shift at the wrong time? I just swapped in a 39 tooth and the shift points seem to be different, I don't have to floor the throttle on a hill just to get the tranny to kick down. Am I dreaming this? If I am not, how come no one mentions calibrating the speedo/tranny to the newbs wanting info on lifts/tires/gears? TIA

there is a speed sensor that inputs to the TCM, but I'm fairly certain it's in the tranny, not tied to the t-case. I know it is this way for the years that used mechanical speedos and I highly doubt they changed it.