Be sure to focus on one pro or con for each paragraph.  If you try to fit too many ideas into each paragraph, your essay will become weak.  The pros are probably fairly obvious.  There would be less damage to the environment and people could save money on fuel.  Many of the cons I can think of are the short-term consequences to fuel efficient vehicles.  Typically, these vehicles are more expensive.  They would also create a dip in the local economy because people would be purchasing less fuel.  Less fuel stops might mean less impulse purchases (like coffee or snacks) from the local gas station. 

Your question made me ask myself whether I have any particular stand in fuel-efficiency vehicles versus traditional vehicles. What came to mind was having learned that that the battery life of a hybrid is not as long as what the media makes it out to be. For this reason, my personal fear is that, while a fuel-efficient car may be able to save you money, what would they do to our environment when they all start "running out"? I mean, what are we doing with  the tons of trash that they will eventually generate? Recycle them? Recycle those big batteries? I admit to being an ignorant in this topic, but it is quite feasible that these little contraptions may not hit the mark and pollute our planet even more. 

I would organize my five paragraphs like this:

1. Introduction (including pertinent background information on the topic) Be sure to include a strong thesis statement that supports your three main points of discussion (your body paragraphs).
2. Financial Pros/Cons.  Consider and compare overall price of the car upfront versus savings in fuel costs later.
3. Environmental Concerns.  Address the true 'green' factor of the car and benefits to the environment.
4. Functionality Pros/Cons.  Is the more fuel efficient car equally efficient in all other domains of consumer use?  Factors could include: speed, horsepower, how often you have to 'charge' it up (if electric), size (some of those cars are tiny and may not be family-size)
5. Conclusion.  Don't forget to restate your thesis here to help wrap up all your major points.

While the pros should be easy, you could research something about how the production of the cars is not all that "green" --especially as related to the battery. I read an interesting article a few years ago that spoke to how much more bio-hazardous those were compared to traditional batteries.

In your introductory paragraph, I would define the term "fuel efficient car" for your audience and explain that you want to compare and contrast five different aspects or qualities of fuel efficient cars today.  Then devote each paragraph to a single topic, giving the "pros" and cons" of that topic.  I don't know what topics you want to use but here are some suggestions:   Alternative fuels vs. fossil fuels, Gasoline vs. diesel fuel, Large vs. small engines, Large vs. small vehicles, and Aerodynamics vs. versatility.  Within each paragraph you could even give examples of certain vehicles that are or aren't fuel efficient to reinforce your point. Use transition words to move smoothly from one paragraph to the next. At the conclusion of your five paragraphs, summarize the important points that you used to inform your audience and let them be the final judge.  Good luck! 

Another possible (though not inevitable) con of having more fuel efficient cars is that they might be lighter and more flimsy than cars have been. 

In addition, if people do not really want fuel efficient cars, the government would be depriving them of their freedom of choice if it pushes for more efficient cars.  It would be essentially requiring them to buy something they do not really want.

Are you looking for cons to creating more fuel efficient cars? I can't think of many, other than perhaps the short-term losses to communities dependent on oil production, though I don't think they'd hurt too much. Another con might be affordability at the front end, i.e., purchase price. Even though fuel-efficient cars save money in the long run, they can be more expensive for the size.