In this area, a relatively small weight reduction can produce significant gains in gas mileage. After market custom wheels are not necessarily bad. Many of them are made of lighter materials than factory wheels and adding a set of custom wheels can be an advantage as long as they are lighter and not much wider than factory stock wheels. Tires, too, should be of the same size as factory stock, or at most one size larger, and no wider than 70 series. Not only will reductions in un-sprung weight give you increased gas mileage, but you should notice a performance improvement as well.
Friction is the second major enemy of good gasoline mileage.
Increase friction and you increase the amount of energy required to achieve the same result. This will result in increased performance and gas mileage. Changing your oil every 3000 to 5000 miles will not only help in reducing friction, but you will get longer life from your motor. Friction in the drive train components will also rob power and increase gas consumption. The aforementioned periodic pressure washing of the chassis and undercarriage will help in this respect too.