Estimating gas mileage: An example of order-of-magnitude physics
  I discuss how to estimate the gas mileage of a car. This discussion, which covers air resistance and Reynolds numbers, describes one way to introduce dimensional analysis and order-of-magnitude physics into introductory physics (if only the syllabus would allow it). It is part teacher's guide and part textbook chapter -- I hope not the worst parts of each. 
Sanjoy Mahajan