Because cars used to be heavy and braking systems were not as efficient as they now are, it was necessary to use the gears (or to be more precise, the engine compression) to help to slow the car down.

因为汽车比较重，刹车系统也不如今天的有效，以前的汽车必须利用变速箱(或更准确地说，引擎压缩)帮助汽车减速。

youdao

Because cars used to be heavy and braking systems were not as efficient as they now are, it was necessary to use the gears (or to be more precise, the engine compression) to help to slow the car down.

因为汽车比较重，刹车系统也不如今天的有效，以前的汽车必须利用变速箱(或更准确地说，引擎压缩)帮助汽车减速。

youdao