If you're at a dealer and are browsing the lot, you can find a vehicle's fuel economy numbers in the EPA section of the window sticker. The EPA label will have three main fuel economy numbers: city, highway and combined mpg.
If the carmakers had their way, they would have you believe that their vehicles always get the highway mpg rating. But in the real world, few cars will actually achieve this number. Most cars will only get this kind of fuel economy on an open road, in perfect conditions and at a certain speed. But in the real world, we drive on both city streets and highways. We get stuck in traffic and we drive in all sorts of temperatures. This is why the combined mpg rating, rather than highway or city, is a more accurate representation of the vehicle's fuel economy. Window stickers on 2013 model-year and newer vehicles are required to have the EPA's new fuel economy label. This label made the combined mpg figure more prominent, as well as adding such useful information as annual fuel cost estimates and greenhouse gas ratings.