[meanjoe75fan]

I've heard it mentioned indirectly, but the reason tractor-trailers are less draggy per weight is known in aerodynamics as the "square-cube rule."

Imagine you take a Metro and double all its dimensions. The vehicle's volume would have gone up 8X (2^3 for 3-D), but the CdA only goes up 4X (2^2 for 2-D). Thus, it would incur 4X the drag of the stock Metro while carrying 8X the load--half the drag per unit of cargo.

This comes into play a lot in aviation, because weight is 3-D, while lift (proportional to wing area) is 2-D. Because of this, it would be possible to build an airplane that's too damn big to leave the ground! (Allowing for current propulsion systems.)

It would be interesting to see if similar classes of private motor vehicles, (i.e. all sedans) with similar HP/weight ratios, become more efficient per pound of cargo capacity as they get bigger...