What is 420g transmission?

As an AI language model, I don't have information on 420g transmission specifically. However, if you're referring to the Tremec T56 six-speed manual transmission, the sixth gear is commonly referred to as the "0.50" or "overdrive" gear and has a 0.50:1 ratio. The T56 transmission has been used in a variety of high-performance vehicles, including the Chevrolet Camaro, Dodge Viper, and Ford Mustang.