The open rules of the Ultimate speed Challenge have led to a variety of interesting car designs., Winning times have improved as wheel technology has advanced and the integration between the cars and wheels has improved via the use of wheel fairings. Wheels play a key role in a car's success in the race. Wheel optimization has included a trend towards a smaller diameter (to reduce inertial effects and aerodynamic drag), the use of custom rubber or urethane tires (to reduce rolling resistance), and the use of solvents to swell the tires (also reducing rolling resistance). There is some overlap in technology between this race and other gravity racing events, including the buggy races race at Carnegie Mellon University.