Two of most aerodynamic cars are the Honda Insight and VW XL-1. Their wheels are basically smooth discs with small vents at the edges for brake cooling. I feel a hubcap of similar design would be pretty effective.
Forum member vman455 took this picture:
I don't personally need and hubcaps, as all of my vehicles have alloy wheels already. I'd put a set on my work van but I don't know if my boss would go for it.