When you have a choice of routes for example for your commute, sometimes choosing the longer one can save you gas overall. A route that lets you drive smoothly at a lower speed without accelerating and stopping will give you better gas mileage. But if the smooth route is a lot longer, it might actually cost you more gas overall, even though you get better MPG. Remember that your real goal is to minimize fuel use, not maximize fuel economy. Of course these two goals are normally the same thing. If you are traveling a fixed distance for example because you don't have a choice of routes then better fuel economy will give you less gas use overall. But when the route or distance itself becomes a variable, then sometimes the shorter route that gives you lower fuel economy could be better. For example, if you have an amphicar, you won't get very good mileage, but shortcutting across a lake could save you so much distance you use less gas overall. Or more realistically, it could be that you can choose to take your commute route over a steep hill. Going over that hill will lower your mileage drastically, but if the distance you save is great enough you could use less gas overall on the commute.
Think about the routes you can take to get where you want to go. Try to pick the one that balances opportunities for smooth, slow travel (use less gas per mile) and cuts down on the total distance. You want to choose the one that uses the least gas along the trip.