Why does the tachometer rise when an automatic transmission car goes downhill?

[McLane]

Automatic transmission car tachometer rise when going downhill is due to gravity acceleration and its own inertia. Situations where engine speed increases: It is a normal phenomenon for the engine speed to increase when an automatic transmission vehicle goes downhill, especially for small-displacement engines, this phenomenon is more obvious. Additionally, this phenomenon often occurs in models equipped with dual-clutch transmissions using multi-plate clutches. When the car is in gear and driving smoothly, the engine's power is transmitted to the transmission, which then transfers it to the drive shaft after speed adjustment, and the drive shaft further transmits it to the wheels, causing them to rotate. Principle of engine speed increase: When going downhill, since the car already has a certain height, under gravity acceleration, the car's descent increases speed. Even without pressing the accelerator, the car's speed is faster than when not pressing the accelerator, meaning the wheels rotate faster. Faster wheel rotation leads to faster rotation of the drive shaft, transmission, and engine, at which point the tachometer shows an increase in engine speed.