It uses "speeds" because it is plural. If it were one car it would say, "Everyone knows it is extremely dangerous to race a car at a speed of more than 280 kilometers an hour."

Also, note that you have the plural for "cars" as well. If you changed "speeds" to "speed", then that means all cars would be raced at exactly the same speed. Here, by "speeds" we are counting possible examples of speed. One car at 310km/h, another car at 290 km/h, and so on.

could be 320 or 310

because there is more than 280 kilometers, you don't know what is the speed, could be anything above 280, then you say speeds

这个问题很有水平。不仔细看很容易受到at speed of +“固定的速度”的影响，仔细想想more than 可能更能说明问题。应该是一个集合，(281.282 283...) 更正一下你的错误，speed可以是countable ，也可以是uncountable。