[ijames]

If those drawings are to scale it looks like when the gap is one tire diameter the Cd has risen about 40% of the total loss, and at 1/2 a tire diameter the Cd has only risen 12% of the total loss. So if the tires are 36" tall (my guess, too lazy to go look), that would be a gap of 18" for the 12% of the total loss, and 36" for the 40% of total loss, which compares pretty well with the 24" max guideline in the article I linked. Thanks, freebeard.