When people drive on freeways, they tend to speed up and slow down constantly, even though it may seem that the speed is being held constant. And it is all these little accelerations and decelerations that can really add up when it comes to your gas bill.
When you are traveling on the freeway, you should start using your cruise control setting more. This will help avoid the slight speeding up and slowing down cycles that drivers tend to do. You should also pull out your manual and find out what speed is best for fuel efficiency in your vehicle, and set your cruise control to that speed.