Robert and Zach live in the same apartment building and work in the same office complex. Consequently, they drive exactly the same route to work every day. However, even though the traffic patterns are basically the same during their respective commutes, Robert gets noticeably better gas mileage than Zach. In exploring why this is so, they note that Robert's car is Brand X and Model A. Robert also takes the car in for regular tune-ups, keeps the tires properly inflated, and always obeys the speed limit. Zach's car is also Brand X, but it is Model B rather than Model A. Like Robert, Zach takes the car in for regular tune-ups, keeps the tires properly inflated, and always obeys the speed limit. What is the most likely cause for Robert's better gas mileage