对白勇语法中一句话的疑惑，与OG冲突

《白勇语法》关于比较的省略中有一句话“比较从句的主语和谓语与主句的主语和谓语相同时，可以全部省去，常常保留作状语的介词短语或状语从句。”

但是OG11-50中却一定要补充“those”，说是“presently  on  the  road”不能与前面并行。

Domestic automobile manufacturers have invested millions of dollars into research to develop cars more gasoline-efficient even than presently on the road.
   

(A)
   into research to develop cars more gasoline-efficient even than presently on the road
   

(C)
    for research for cars to be developed that are more gasoline-efficient even than presently the road

记得还有大全里有一题也是一定要补齐“it  was”。

而OG11-102却是可以只保留状语，把主语谓语都省略了。

198. Although Napoleon’s army entered Russia with far more supplies than they had in their previous campaigns, it had provisions for only twenty-four days.

(E) for any previous campaign

到底是在什么情况下能省略？什么情况下就一定要补齐主语、或者主语谓语？

presently on the road不是介词短语或状语从句,即使是主从句作比较也不能完全省略主谓。

何况你举的例子是补语的比较不是句子的比较。