[basjoos]

The main reason that lowering a vehicle improves mileage is that, in conjunction with installing a front airdam, it diverts the air around the draggy underside and it exposes less of the wheels to the oncoming airflow, thus slightly reducing the frontal area. If you raise a vehicle that has a smooth underpanel installed, then the only drag increasing effect is exposing more the wheels to the oncoming airflow and increasing the frontal area. Raising a vehicle reduces the drag of the air passing around the vehicle since the air is displaced less as it splits around the vehicle. Instead of air being able to pass only around the top and sides of the vehicle, the air can pass around the top, sides,and bottom of the vehicle, so it is displaced less as it splits in front of and then recombines behind the vehicle. If you look at a lot of the ultra high efficiency solar challenger type vehicles, they raise the vehicle's body very high above the road and use skinny wheels with fairings to reduce the drag increase from the tall wheels. They, in essence, turn the vehicle's body into a low flying aircraft fuselage supported on tall, thin wheels. To get the same effect on your truck, you would need to keep the wheels skinny (think of the skinny tires on the mail delivery trucks of the past) and install fairings on front and behind the wheels to divert the air flow around them.

On my vehicle, it do this to some extent as I have a smooth underpanel and have enclosed my wheels in a pontoon that diverts the air around the wheels for the length of the car. My vehicle's nose has a low stagnation point so most of the wind goes over and around the sides of the vehicle, but I have arranged for a good portion of the wind to go under my vehicle where it helps to fill in the area under the boattail at the back. It also allows possums and other small animals to pass untouched underneath my vehicle, which wouldn't be the case with a lowered, airdam equipped vehicle.