An automatic transmission decides when to shift many times a minute. The decision uses more inputs than road speed, which is why the same speed produces different gears.
The basic trade-off is torque against efficiency
A lower gear multiplies engine torque, giving acceleration. A higher gear reduces engine speed for a given road speed, reducing fuel consumption and noise.
The transmission is continuously choosing between these, seeking the highest gear that still delivers the acceleration the driver appears to be asking for.
That is why a car cruising gently sits in a tall gear and drops several gears the moment the accelerator is pressed firmly.
Throttle position expresses intent
How far and how quickly the accelerator is pressed is the main signal of what the driver wants, and it is weighted more heavily than the pedal's absolute position.
A rapid press is read as a request for acceleration and triggers a downshift; the same position reached slowly may not, because the rate suggested no urgency.
Shift points are stored as maps relating throttle and speed to gear, and the transmission moves between different maps depending on how the car is being driven.
Other inputs modify the decision
Road gradient is inferred by comparing the acceleration the engine should be producing against what the vehicle actually achieves. A shortfall implies a climb.
On a descent the transmission may hold a lower gear deliberately, using engine braking so the driver does not have to hold the brakes continuously.
Fluid temperature, engine temperature and even steering angle can influence shift behaviour, since shifting mid-corner is undesirable for stability.
Shifts must be managed, not merely commanded
Changing gear requires releasing one clutch pack while engaging another, and the overlap between them determines whether the shift is smooth or abrupt.
Hydraulic pressure is modulated during this overlap, and engine torque is briefly reduced through the engine controller so the driveline is not shocked.
This coordination between transmission and engine control is why the two systems communicate constantly rather than operating independently.
Adaptation happens over time
Control software adjusts its pressure and timing values as clutch material wears, so shift quality is maintained rather than degrading steadily.
Some systems also adapt shift points to observed driving style, holding gears longer for a driver who accelerates hard and shifting earlier for one who does not.
This is why a transmission can feel different after a period of unusual use, and why the behaviour settles back once ordinary driving resumes.