Steam. Although the 1906 Stanley race car was much more streamlined than other race cars of its time, the car was steam powered. Its steam engine and boiler arrangement allowed it to have a streamlined cigar shape that coould not be achieved by cars with more typical engines.
Cars with good aerodynamics.
Their shape and aerodynamic efficiency makes them streamlined.
aircrafts and fast cars are streamlined so that the air flows over them more easily
Ferrari
They have a streamlined bonnet so that the air will be pushed over the car.
If race cars didn't race they wouldn't be race cars.
Helps improve fuel mileage.
the way they were desinged by the builders
to be able to get through water as easily as possible, the same way that planes and fast cars have to be streamlined.
less wind resistance, better fuel mileage
The most common features in modern cars that help to reduce air friction is the streamlined design of the bumpers and spoilers. The streamlined shape of fast cars also helps to reduce drag.