Futurepundit notes that
replacing half the ferrous metals in current automobiles could reduce a vehicle’s weight by 60 percent and fuel consumption by 30 percent, according to some studies. The resulting gains in fuel efficiency, made in part because smaller engines could be used with lighter vehicles, would also reduce greenhouse gas and other emissions by 10 percent to 20 percent..
All of this would come with no sacrifice in safety, as preliminary results of computer crash simulations show that cars made from carbon fiber would be just as safe – perhaps even safer – than today’s automobiles. Today’s Formula 1 racers are required by mandate to be made from carbon fiber to meet safety requirements.
Other forms of transportation:
Segways can get the equivalent of 450mpg based on the amount of gas it would take to create the electricity needed to run it
Folding electric bikes do even better and can be taken onto public transit at all times One could use composites or magnesium for the body and lithium ion for the batteries and get the bikes down to 15 lbs or less. You also would not need a 10 pound lock since you could fold the bike and take it to your office or whereever you are going.