Fastest I've driven was 160km/h in a 92 ford escort. It sounded like it was going to fall apart, and the tires were only rated to like 120 km/h
I got a ride in a skyline that got up to 245km/h before he had to let off because of a bend. It was still pulling hard, so I don't think 300km/h would have been out of the question had the road not curved.