It isn't really car-length, is it speed-adjusted to scale larger at high speeds and closer at low speeds. But the display on the screen shows a car with a line

It isn't really car-length, is it speed-adjusted to scale larger at high speeds and closer at low speeds. But the display on the screen shows a car with a line and the numbers 1-7 as you adjust it so people shorthand refer to it as "car-lengths"