I obtained a kids ride on toy motorcycle for free for my kid. It was well used and came without a battery. After placing a 12v in it, I discovered that it ended up too fast and jerky for my kid to ride it enjoyably. So this is a video of how to modify a kids ride on bike with rubber tires and a speed control to have a smoother ride.

The rubber wheels were purchased from Harbor Freight. The speed controller is a DC motor PWM module from online. There was some electronics patchwork that I made to interface the existing motorcycle's pedal to the speed controller to turn it into a throttle.