Modern cars are made from a variety of materials. The materials chosen depend a lot on their cost, repairability, and durability. Fuel efficiency is just one of many variables on the minds of car designers.

The typical family sedan these days is usually made from steel body panels with plastic front and rear fascias. Some of them may have an aluminum hood and rear deck lid for weight savings. The unibody (the underlying frame) is almost always made from a combination of steel and special (and costly) high-strength steel.

Let's take a look for a moment at the unibody. It has a number of jobs to do, including absorbing and distributing crash loads, serving as the mounting skeleton for the body panels and drivetrain/suspension, and providing structural rigidity to the vehicle. Structural rigidity affects a number of aspects of the driving experience, including NVH levels (noise, vibration, and harshness) and the handling of the vehicle (by providing a solid mounting point for the suspension components). Post-crash repair is an important consideration as well, as complex manufacturing methods (such as bonding aluminum with exotic epoxies, for example) are extremely hard to replicate in your neighborhood body shop.

The body panels must consider not only weight and cost, but other factors such as the ease of manufacturing and repair. Paintless Dent Removal is nearly impossible to do on aluminum, for example, due to its limited memory properties. Complex shapes are easier to form into some materials than others, as well, so the designer's pen and the manufacturer's existing tooling has a hand in the choice.

Carbon fiber is exceptionally difficult to mass produce. The BMW M series recently got the first application of machine-made carbon fiber for use in a roof panel, and it was considered a breakthrough. It also requires a special clearcoat paint to protect it from UV damage.

The plastic exterior panels famously employed by Saturn in the 1990s had mixed results and were abandoned. (The panels would sometimes shatter instead of flex, for example, when met with an impact.)

Fiberglass, or a composite material, is employed in some vehicles such as the Chevrolet Corvette, and my last car the Lotus Elise. The Lotus body panels were made from a combination of machine-formed and hand-formed composite, which is kind of like an exotic and thin fiberglass. This material was fairly brittle, and extremely expensive and difficult to repair. If you have a track car where weight savings are of the utmost importance, it might be worthwhile to build with this material. If you have a Corolla that will see a lot of commuter duty in the land of speed bumps and door dings, you may want to lose an MPG or two and go for steel.