Why do some cars have a higher fuel efficiency than others?

I've noticed that some cars seem to use less gas or have a better mpg than others even though they are similar in size and weight. What determines a car's fuel efficiency and how can I improve it?

The main factor affecting fuel efficiency is the car's engine. Cars with smaller and more efficient engines typically use less fuel. Additionally, other factors such as aerodynamics and weight also play a role. To improve fuel efficiency, you can try to drive more smoothly, avoid idling for long periods, and keep your tires properly inflated.