This section describes an optional methodology for determining improved aerodynamic drag area, CdA, for vocational vehicles, as described in § 1037.520(m), rather than using the assigned values. A vocational vehicle's aerodynamic performance is based on a DCdA value relative to a baseline vehicle. Determine a DdA value by performing A to B testing as follows:

(a) Use any of the procedures described in this subpart, with appropriate adjustments, for calculating drag area.

(b) Determine a baseline CdA value for a vehicle representing a production configuration without the aerodynamic improvement. Repeat this testing and measure CdA for a vehicle with the improved aerodynamic design.

(c) Use good engineering judgment to perform paired tests that accurately demonstrate the reduction in aerodynamic drag associated with the improved design.

(d) Measure CdA in m 2 to two decimal places. Calculate DdA by subtracting the drag area for the test vehicle from the drag area for the baseline vehicle.