Correct answer is... 474 km/h. It is equal to 256 kn or 295 mph. It is higher value then max speed of other aircrafts of a similar size. Average is about 290 km/h (156 kn or 180 mph).
The maximum speed most frequently is a term for so-called never exceed speed. Above this speed, the Cessna 421 can suffer considerable damage. There are also three other types of speed that can be called "maximum speed": Vno (normal operation speed) is the maximum allowable speed of normal operation, Vb (design speed for maximum gust intensity) is the maximum speed in rough air, and Va (design maneuvering speed) is the maximum speed at which the rudders can be maximally deflected.
Disclaimer: Our articles and data connected to max speed of Cessna 421 are based on public data from various sources. We are very picky to select an reliable data providers, but we cannot guarantee that everything is 100% error-proof. We recommend for any serious purpose - as a rule of thumb - to request offical data about max speed from manufacturer of Cessna 421.