What is the difference between "city miles" and "highway miles"?

What is the difference between "city miles" and "highway miles"?

Terry Tamminen (Author & Environmental Policy Advisor) gives expert video advice on: Why is fuel efficiency important to the environment?; How does the weight of my car affect fuel efficiency? and more...

Well city mileage is usually worse than highway because when you're on the highway, you're thought to be traveling at a fairly consistent speed. The inefficiencies of the internal combustion engine are a little bit less noticeable because when you're in start and stop traffic in the city, you're accelerating. You're taking that heavy machine from a dead stop and having to move it. Imagine if you were doing this just with your own body power and trying to move a car from a dead stop as opposed to just pushing it a little bit more once it was already rolling. Well, gasoline has the same effect, it's not very efficient when it's trying to move that car from a dead stop or from a slow pace, than if it's already rolling down a highway.