Passive voice is not the real problem here. In the first sentence, "To increase efficiency" is followed by "more cars." It is unclear who is trying to increase effiency. The second version fixes this problem by putting "the company" after the opening phrase.
As for the other sentence, neither version works well. "Due to" means "resulting from," and neither "we" nor "less spending" can be said to be resulting from the higher costs. Use "because of" here, since "because" can modify verbs and clauses.
Because of higher costs, we need to reduce spending.
The original sentence would not work this way. If we say "Because of higher costs, less spending is needed," it sounds as if the higher costs actually reduce the
need for spending, when actually they are making a reduction necessary.