Cars have improved upon fuel efficiency greatly over the past 40 years. In the 1970's, gasoline in the US was in the 50 cents per gallon range (excluding any gas shortage effects in the late 70's), so filling up the gas tank was in inexpensive endeavor and nobody really paid much attention to fuel consumption patterns and how to improve them. Fast forward 40 years and fuel prices are considerably higher, to the point that fuel economy is a very important factor to most people when owning a car. Unfortunately, gasoline based car engines are inherently inefficient, maxing out at about 25-30% at best. Most of the energy from burning the fuel is lost as heat and noise to the atmosphere. But serious improvements in fuel efficiency have most certainly come about in recent years.
Technological improvements in fuel efficiency have come about due to changes to both the engine and to other parts of the car. First of all, modern computers have greatly reduced the amount of time it takes to engineer a better engine design. In the past, prototype engines had to be build to test new designs based on mathematical calculations. Now, computer simulations can help narrow down design changes and choices much more quickly before entering the time consuming prototype building stage. So changes in things like cylinder arrangement, piston bore diameter, stroke displacement, etc. have improved fuel economy greatly in recent years.
Other changes have improved fuel efficiency as well. One of them is reducing weight. As cars buyers have expected more fancy and high tech options on their cars over the years, weight also increased as well. In addition, standard safety features found on all cars today also add to the weight of the car. This has been counteracted by reducing the weight of body components by using composite polymer materials instead of metal. Add in sleeker aerodynamic profiles to reduce wind resistance, improving cooling systems to help the engines run less hot, and improving tire quality and engineering to reduce the friction between the tire and the road under normal dry driving conditions, and all of these technological improvements add up to serious increases in fuel efficiency over the years. Future improvements in all of these areas will continue to help in years to come.