You can picture a row of gears standing one above another. The bottom ones give power for starting and climbing; each step up power for speed. Top gear is the last step, the one you reach on an open road, where the engine turns slowly and the vehicle runs fast and easy. That top step is the whole word: the setting for going as fast as this machine can go.
可以想成一排由低到高的檔位。最下面幾檔是用來起步和爬坡的，力氣大、速度慢。每往上一格，就用一點力氣換一點速度。top gear 是最上面那一格，只有在路開闊、車已經跑起來時才用得到；這時引擎轉得慢，車子卻跑得又快又輕鬆。整個詞的畫面就停在那一格上：這台機器能跑到的最快設定。