Most gasoline based sedans have between 3 and 6 cylinders and the mileage per gallon (highway) varies between 25 and 40 and can carry around 5 passengers.
A sedan carries upto a 1000 pounds more when carrying 5 passengers instead of a single passenger.
With the advances in technology, new car engines could be designed that would switch between the number of cylinders being utilized, based on the number of passengers in the car. Such cars would run with higher mileage when only a single passenger is in car.
A concept similar to this one, has been tried previously, though not very successfully.