Speed based animation

I recently tried to animate a car and trying to control the speed of it can be very tricky and time consuming, so I would add some sort of modifier to the graph editor that uses the normal graph editor input as the “speed” of the graph.
So instead of inputting value over time, you would input change of value over time.
This could also be done in both ways.
Seems like a fairly simple feature that could making animation a lot easier.