Hot Wheels released the Revvers series in 1973 only. They were diecast vehicles that were slightly larger than regular Hot Wheels and when the wheel located at the back of the car was wound up, a button on the bottom would release the tension and they would "rev" along the floor under their own power. The wheel was connected to a rubber band which was connected to a small array of plastic gears connected to the rear axle. It was similar to that of a wooden airplane. There were several models released, including some in two-packs, and even one that resembled a camping trailer.