The students hear about "fuel efficiency" in cars all the time.  However, most of them don't connect this to the term "efficiency" in their science classes.  I like to ask them what makes a car more fuel efficient.  They will often know that the car's size, shape, and engine type will affect the fuel efficiency.  This leads us into discussions about energy conversion, air resistance, and Newton's second law.  They start to see that as you lose work that the engine is putting in, you aren't getting the most work out of your car for your gas dollar.