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Wind tunnel tests are conducted using l/8th scale models of a standard U.S. mini-van to study the effect of inter-vehicle spacing on the drag of each of the vehicles in 2, 3 4-car platoons. The wind tunnel has a ground plane equipped with boundary layer control provided by means of suction over a homogeneously porous surface. Internal, 3-component force sensors are used to measure drag, side force, and yawing moment on each model. Drag coefficient of an individual vehicle alone is about 0.33. When spacing between platoon members is greater than about one vehicle length, the interaction among members is relatively weak. The drag of the lead vehicle is unaffected by the presence of other platoon members; each succeeding vehicle exhibits a lower drag coefficient. For spacings less than 1 vehicle length, drag coefficients are reduced for all members of the platoon, although the interior members experience the greater savings. At a spacing of half a vehicle length, the average drag coefficient in a 4-vehicle platoon falls to 0.62 of the non-platoon value.
Zabat et al. (1994) studied this question.