GRX

The GRX was technically an engine, but it has become more identified with the gold-colored car that housed the engine in the series episode "The Fastest Car on Earth." The engine was designed by Ben Cranem, and was responsible for crashes and deaths of four test drivers and its inventor due to the impossible speeds it could attain. Cranem died and the GRX engine was buried with him, but Oriana Flub and her men exhumed the engine and placed it into the car with a sleek, golden, and markless body. Oriana convinced Speed to test drive the car with the GRX and Speed was sprayed with a special serum known as the V-gas to artificially sharpen his reflexes. 

The V-gas causes its driver to become extremely thirsty and if the driver consumed any compound containing water, it would tamper with his mentality and would develop a strong phobia of speed. Like the other test drivers who were given the gas, Speed became scared of even the slowest of speeds. However he regains his love of racing due to Pops' help in the race that followed. By then, the car had a new driver, Cranem's son, Curly. Curly was given the V-gas and soon experienced its side effects. 

The GRX and its engine were destroyed when he fatally crashed the car due to Curly drinking water during a pit stop negating the effects of the V-gas. The GRX episodes mark one of the few continuity errors introduced by the English dubbing. In the first episode the GRX's speedometer with a maximum speed of 400 kilometres per hour (250 mph) on it is shown in the beginning of the episode, however, due to a continuity error in the Japanese animation, as Speed drives it, the speedometer tops out at 440 kilometres per hour (270 mph) This would make the GRX slower than the Mammoth Car by the English dialog. In the 2008 film adaptation, the name makes an appearance as a car developed by Royalton Industries and driven by Jack "Cannonball" Taylor.

 The car retains none of the back story from its anime counterpart, and is redesigned into the racing competition of the film. It is numbered 66 and colored purple and gold and was transformed from a two-seater to a single-seater. In the Grand Prix race that closes the film, the GRX is the main competitor for Speed in the Mach 6 and features a secret weapon called a "spear-hook" that is illegal in professional racing. After Taylor deploys the device against Speed during the Grand Prix, Speed uses the Mach 6's auto-jacks to flip the cars and reveal the hook to the track cameras, automatically disqualifying Taylor and aiding the case built by Inspector Detector against Royalton.

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