My air dam outperformed my belly pan ('94 Geo Metro) so far.

I wonder if you really need to go that low to the ground since air at the road surface should not impact the drag on the vehicle. Numbers I heard was 2.7 inches for the clearance for a car - I would think that a truck with more ground clearance could tolerate an even higher air dam since the underbody is so high up anyway. You may be adding more drag by blocking too much of the air with one that big and close to the ground.

In response to JanGeo, I have come to the conclusion that about 1.5 inch clearance above grade is about right for my truck.

First of all, that?s where road rubbing in low-speed situations ends. Mostly mine rubbed the ground under braking until it wore the dam to a 1.5 inch clearance.

Second, aerodynamic studies I?ve seen on pickups suggests that the lower clearance the better. There will be a boundary layer starting at the road surface and slowly getting thicker along the length of the truck. Pickups are long and they give that boundary layer a chance to contact the bottom of the truck. For short-wheelbase/length vehicles, maybe a 2-4 inch clearance may be A-OK.

Third, I need access to some parts of the underneath area to service the truck and that rather precludes belly pans. I may be able to incorporate a belly pan aft of the rear axle and that?s where I would fear a boundary layer building up enough to contact the truck.

It occurs to me that the reason that this airdam appears to be successful is that it is right at the front of the vehicle, it's not too far back under the bumper. This allows some of the air to escape up over the hood as opposed to only around the sides. When an airdam is too far back under the bumper, air cannot go backwards against the direction of travel to escape over the hood, it either goes out the sides, or the pressure is great enough to force it under the car anyway. If the location of the airdam further forward is problematic, a lip at the base of the airdam may stop air forcing under the bottom, and encourage to escape over the top. A lip may also enhance efficiency of an optimally located airdam.