[island911]

. . . you can measure the combination (Aerodynamic Drag and Rolling Resistance) by doing a "Roll-down" test.

This is where you map (plot) the deceleration from coasting on a flat road. (or coast both ways on an almost flat road, and average the two. . .may account for wind, that way too)

If you point a camcorder on the (accurate) speedo, you can easly plot the deceleration (speed/time). Then, throw in the weight (mass) of the car, for the rolling & drag force combo. (F=ma)

edit; . . and the 'force thru distance' is work (HP) . . .the area under any section of the ploted curve (integral) will be equal to the distance traveled for that section.

That's my shot at a layman explaination.