Of course Titanium and aluminum would be best for the fuel economy ratings of a given car; however, the efficiency of the car driving down the road is only one small part of the fuel consumption story. There is also fuel consumed in the manufacturing and de-manufacturing process.

Steel takes relatively little energy to produce compared to aluminum and titanium. And I believe it takes even less to "recycle" when the car has gone to the great autoban in the sky.

If you really want better fuel efficiency, strip the interior of YOUR car and install folding lawn chairs. Think of all the weight that those seat cushions and other unneccessary items like radios, take up.

And oh yeah, your fuel efficiency would increase if you would lose some weight...