what is city/highway driving?

City mileage, as you suggest, refers to stop and go traffic: the type of driving you expect to encounter in any city of really any size as long as it has stop signs, traffic lights, etc. It is the type of driving where you cannot stay at a consistent speed for very long. Of course, this type of driving is typically worse for gas mileage because of the constant stopping/going and possible cutting on/off of the engine. The more smoothly you can drive the transitions from stop and go, the better your mileage. IOW, jackrabbit starting and slamming on the brakes is hard on the car and will hinder your mileage.

Highway mileage is not restricted to interstate only, but certainly refers to the type of driving here you can maintain a consistent speed with little to no stops (the type of driving where the use of cruise control is optimal). The actual speed itself doesn't really matter, although the higher your speed, the more fuel you will expend.

 

I'm not sure I understand your question. If you're asking about EPA test cycles and methodology, see http://priuschat.com/forums/other-c...uth-about-epa-city-highway-mpg-estimates.html.