破折号后应该是充当a speed of forty miles an hour的定语的.
完整句子应该是The winning car reached a speed of forty miles an hour which is faster than any of its rivals.这里加了破折号句式就可以灵活,不用刻板地一定要用定语从句的引导词了,这一点我讲不清为什么能够省略,你可以参照一下别人的回答.
至于比较级,是跟a speed of forty miles an hour构成的咯.which在定语从句中代替a speed of forty miles an hour作主语,定语从句中与its rivals进行比较.所以其实这里还省略了,更完整的句子应该是The winning car reached a speed of forty miles an hour which is faster than any of its rivals‘ speed.
那个,感觉充当car的定语也可以解释得通.