It depends on the vehicle really, like with mopar for instance I know they set their computers not to release full power to the motor until 150hrs have been put on the motor, and after the hrs have been on there you get like 5-10 better both city and highway. Driving habbitts really bertermine a lot too, NHTSA says that when you cruise at a speed higher than 60-65 MPH your MPG drops drastically for every MPH over. Anywho also any kind of air filter upgrade you can make really helps, and also about every 2 - 3 fillups use a full tank of 89 octane, if you're using 87, don't use 93 unless your owners manual calls for you to. If you use 89 consistently you should see an increase 3-5 MPG better.