He wants Wal-Mart's fleet of more than 7,000 trucks to get twice as many miles per gallon by 2025.

卡车每加仑可以走双倍的距离。

看语法书里说，倍数+as...as,表示...是...的几倍，比如listen twice as much as we speak.第一个as是副词，第二个as是连词，但是这个句子为什么只有一个as,没有第二个as的对象它跟谁比较呢？比较对象是谁呢？谢谢老师。