This depends on a lot of things- what are your driving habits (leadfoot?), what grade of fuel are you using, lots of short trips or mostly highway, etc. Assuming you're driving a good mix of city and highway, not drag racing everybody that pulls up next to you, and are using at least 89 octane, check your tire pressure first. Then put the car on jackstands and check to see that your brakes aren't dragging. Next, if all that checks out, if you have a tachometer, count the shifts while you're driving to make sure overdrive is kicking in and that your converter is locking and unlocking properly. You might think about weight, too. You'd be surprised what a difference removing 100 lbs. from the car in the form of trunk-junk, etc. can make.