The tzero is a handmade electric sports car designed and built in very limited numbers by the U.S. company AC Propulsion. The tzero was based on the Piontek Sportech kit car, which consists of a fiberglass body built over a reinforced steel space frame with double wishbone independent suspension and rack and pinion steering. AC Propulsion added the AC-150 drivetrain, a single-speed electric system with an overall gear ratio of 9:1. Launched in January 1997, only three prototypes were built and plans for commercial production were dropped in mid-2003. The name comes from t₀, the mathematical symbol for a starting point in time. Due to high production costs, AC Propulsion ceased to produce the tzero. Only three were built, one of which is owned by the company itself, one by Gruber Power Services, a company that specializes in Tesla Roadster repair, and one privately. The drivetrain, however, was used in many other vehicles. Because the car recharges its batteries when the throttle is released — slowing sharply as energy is recaptured — it can be driven hard using only the accelerator pedal. Also, if the car detects a turn with more than half a g-force (5 m/s²), it eases the rear-wheel regenerative braking to prevent slides.