[MetroMPG]

I went looking for a source to quote, but didn't find one.

But I thought the rule for flow rate through an orifice (gap under the front of the vehicle) is that it's effectively proportional to orifice size for a given pressure.

And when you consider that dropping car closer and closer to the road may actually be increasing pressure at the air dam as the gap shrinks, the effect may be even more pronounced.