In 1967, Gyro Transportation Systems engineered a two wheeled inline vehicle named the Gyro-X. Designed by Alex Tremulis the Gyro-X balanced itself by using a 20 inch hydraulic gyroscope. Although the car never made it to production it was said to reach a top speed of 125 mph.
Watch a Video of the Gyro-X in Action:
Photo and Video Courtesy of Steve Tremulis and Thrustcycle