But aircraft that excel at subsonic flight are inefficient at mach speeds and vice versa .
然而善于亚音速飞行的飞机，在进行超音速飞行的时候效果是不理想的，反之亦然。
They excel at ironing out the volatility between city and highway driving efficiency and they have proved extremely reliable over the long run ( though they command a price premium over their conventional counterparts ) .