The leaky old windows made way for more efficient ones.

Burlington Northern will use the engines to replace less efficient ones.

But my guess is he saw her as another of his machines-one of the more efficient ones.

Replacing older fluorescent lamps with newer, more efficient ones would help, too.

It seemed inevitable, as gas began its rise, that the new fuel would be substituted for older, less efficient ones.

This is mostly due to older vehicles being replaced with newer, more fuel efficient ones.

In terms of practical security, a major concern about these new attacks is that they might pave the way to more efficient ones.

The polynomials commonly applied are not the most efficient ones possible.

They, in fact, need fewer but more efficient ones.

In effect, the quick closing of inefficient plants will allow energy companies to use efficient ones longer.