That's the max in KM/H , you just turn it to MPH and then multiply it to 1.6 and it will be your speed in KM. The reason the speed is showing 399km is that because in the first years of the game, there weren't cars that could even go 380km, but now there are many, but they still didn't change the limiter , there is no limit for speed
First of all this is a special edition D spec M3 E36, if you have the same car and your speed is 286km/h , change your gearbox ratio, or make your final drive close to 2