The Race to 100 MPG - Popular Science
See the article above for a few ideas about how to make cars that can do 100 miles per gallon (to go back to Imperial for the moment.)
Interestingly, the X Prize people are going to get involved in this too:
The race should heat up further when the X Prize FoundationÂthe group that kick-started the space-tourism industry with its $10-million competition to produce a reusable private spacecraftÂannounces in the next few months a competition for the first car to break 100 miles per gallon and sell a yet-to-be-decided number of units. The prize money hadn't been finalized at press time, but X Prize officials are discussing figures in the $25-million range as an appropriate incentive. They hope the prize will urge people to completely reconsider what a car should look like and how it should function. 'We need a paradigm shift,' says Mark Goodstein, the executive director for the automotive X Prize. 'We need to change the way people think about automobiles.'
The photo of a prototype super lightweight and aerodynamic car at the top of the article gives an idea of how odd such a vehicle may look. One thing that crosses my mind as soon as I see that picture is how it looks like it could double as a pizza oven if it was parked in the Brisbane summer sun for more than 5 minutes. Look at that big oblique windscreen that seems designed to let as much heat as possible.
Don't forget such obvious practicalities, bug car designers...