That's 186 mph. In the US, the highest posted speed limit anywhere is 85 mph, on some stretches of highway in remote rural Texas. So you're looking at more than 2x the speed limit of the highest posted speed limit in the country. A lot of cars come preprogrammed with some speed governor in them. They usually top out at an already absurd speed of 125-155 mph.
So 300 km/hr is absolutely insane. That's enough to get jail time in most states.
Oh, and for added physics fun, that extra 40 mph represents a 62% increase in vehicle kinetic energy and stopping distance.