The GRX gets its name from a type of engine in the original Speed Racer (1967) series (but originally called ER-X in the original manga) and makes its debut in the episode The Fastest Car on Earth (Part 1). This engine propels the car so fast that it was responsible for the death of several test drivers.
Originally designed by Bent Cranium, the GRX engine simply proved too fast. After taking the lives of 5 test drivers and eventually its creator, the GRX engine was burried with Bent Cranium. Exhumed by Oriana Flub for use in the Oriental Grand Prix, the golden GRX was built. To combat the hallucinations and trauma caused by the massive speeds the car reaches, Oriana Flub develops a special gas. After The GRX nearly takes Speed Racer's life and sanity several times, the day of the Oriental Grand Prix finally comes. The GRX's new driver, Curly loses control after water interferes with the gas and causes him to lose control and have a hideous crash.
In the Speed Racer Film the GRX is Royalton Industries #66 GRX driven by Jack "Cannonball" Taylor. Outfitted with the standard Auto-Jacks, the car features a motor built in collaboration with Musha Motors. Already a fierce car, it became even fiercer as Arnold Royalton secretly installed a "spear hook" on the car for the final race of the season, the WRL Grand Prix. In the end however, the GRX and "Cannonball" Taylor failed to beat Speed Racer let alone take him down, the GRX and Cannonball Taylor were both disqualified from the race. During the movie, it is confirmed by Royalton that the GRX is able to reach speeds of 800 KP/H(497.20 MPH).