How could I make a custom camera based on the HumanoidRootPart velocity?

based on the speed of a part? what?

You need to elaborate. If you want the camera to follow a moving object, use the CFrame.new(position,pointToThisPart) method within a loop or a RenderStepped event-triggered function.

I know the basics of making a custom cam, I just need to know how to make it based on the Velocity property of a part,

I'm really not sure why you would want to base a camera on the velocity property when you can use other properties to achieve the same result. What's the reason you want to base it on velocity instead of CFrame?

CFrame always rotates the camera when the object itself rotates, and the Velocity property doesn't. Basically, I just want my camera to behave like the Asphalt 8 camera when your car does air stunts.

The request seems unclear, and it would be much appreciated if you could elaborate on what you mean by "based on Velocity", or describe how the camera in Asphalt 8 behaves.

So basically I want it to use the Velocity property of a part instead of the CFrame property of a part. The cam in Asphalt 8 basically rotates with your car, but only on land.