What derivative gain does
The cruise controller compares actual speed with the set speed and adjusts the drive-by-wire throttle to close the gap. Proportional gain reacts to how big the gap is. Integral gain reacts to how long it's been there. Derivative gain reacts to how fast the gap is changing.
When the car is closing in on the set speed quickly, derivative gain pulls throttle back. That slows the approach and reduces the chance of overshooting the target.
Why it matters
More derivative gain takes away more throttle during that approach. That damping lets you run higher proportional and integral gains before the controller becomes unstable, which usually means a faster, firmer response to hills and set-speed changes.
Too much derivative gain near the target can cause a subtle, repeating surge. Passengers feel it, and it can make them carsick. If the car gently rocks at a steady speed, back derivative gain off.
Units
The values in this table depend on the speed units selected in Preferences. If you switch between mph and km/h, expect the numbers to change and recheck the behavior.
Wiring and the rest of the function are in cruise control wiring and cruise control.
