請問 a per passenger mile basis?

1.請問一下各位soon在下面這句要怎麼翻呢
2.a per passenger mile basis這麼會 a 加 介係詞呢?
還請各位解惑
Its engines will be quiet enough to meet the strict new airport noise standards now or soon to be in place in many countries,and when measured on a per passenger mile basis will be more fuel efficient than the average family car.
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