If you must use a car, then you should at least try to use it as efficiently as possible. Jalopnik has some simple suggestions for improving mileage. Obviously, I would have put “Don’t use your car when you don’t have to” at the top of the list, but I don’t have a car, so it’s my only option.
Should I ever need to get a car I’ll be looking for something I can run on biodiesel, or possibly a biodiesel/ petroleum diesel mix because you can’t always get to a green fuel pump. I hanker for a Volvo estate or small van to cart my bikes around in. Efficiency isn’t such a concern with a closed circuit fuel supply like biodiesel, but VCACarFuelData.org.uk, allows you to search for your next ride by this and other criteria. If I could forgo the space considerations then the Citroen C1 would be worth considering, the most efficient of the cars currently listed on there.
Of course, another option is to do something as inventive as this Russian driver, who replaced his Opel’s engine with a 50kw motor and filled all the spare space with batteries.