在车上写作业的英语表达是 "do homework"。这个短语简洁明了，直接传达了“做作业”的意思。如果你想要更具体地描述在车上的动作，可以使用 "doing homework in the car" 或 "writing homework while driving"。不过，通常情况下，"do homework" 就足够表达在车上写作业的意思了。

例如：

1. I need to do my homework before we go to bed.

2. She was doing her homework in the car on the way to school.

希望这些信息对你有所帮助!