(a) Where applicable, a vehicle's roof height and a trailer's length are determined from nominal design specifications, as provided in this section. Specify design values for roof height and trailer length to the nearest inch.

(b) Base roof height on fully inflated tires having a static loaded radius equal to the arithmetic mean of the largest and smallest static loaded radius of tires you offer or a standard tire we approve.

(c) Base trailer length on the outer dimensions of the load-carrying structure. Do not include aerodynamic devices or HVAC units.

(d) The nominal design specifications must be within the range of the actual values from production vehicles considering normal production variability. In the case of roof height, use the mean tire radius specified in paragraph (b) of this section. If after production begins it is determined that your nominal design specifications do not represent production vehicles, we may require you to amend your application for certification under §1037.225.

(e) If your vehicle is equipped with an adjustable roof fairing, measure the roof height with the fairing in its lowest setting.

(f) For any provisions in this part that depend on the number of axles on a vehicle, include lift axles or any other installed axles that can be used to carry the vehicle's weight while in motion.

(g) The standards and other provisions of this part apply to specific vehicle service classes for tractors and vocational vehicles as follows:

(1) Phase 1 and Phase 2 tractors are divided based on GVWR into Class 7 tractors and Class 8 tractors. Where provisions apply to both tractors and vocational vehicles, Class 7 tractors are considered “Medium HDV” and Class 8 tractors are considered “Heavy HDV”.

(2) Phase 1 vocational vehicles are divided based on GVWR. “Light HDV” includes Class 2b through Class 5 vehicles; “Medium HDV includes Class 6 and Class 7 vehicles; and “Heavy HDV includes Class 8 vehicles.

(3) Phase 2 vocational vehicles with spark-ignition engines are divided based on GVWR. “Light HDV” includes Class 2b through Class 5 vehicles, and “Medium HDV includes Class 6 through Class 8 vehicles.

(4) Phase 2 vocational vehicles with compression-ignition engines are divided as follows:

(i) Class 2b through Class 5 vehicles are considered “Light HDV”.

(ii) Class 6 through 8 vehicles are considered “Heavy HDV” if the installed engine's primary intended service class is heavy heavy-duty (see 40 CFR 1036.140). All other Class 6 through Class 8 vehicles are considered “Medium HDV”.

(5) In certain circumstances, you may certify vehicles to standards that apply for a different vehicle service class. For example, see §§1037.105(g) and 1037.106(f). If you optionally certify vehicles to different standards, those vehicles are subject to all the regulatory requirements as if the standards were mandatory.

(h) Use good engineering judgment to identify the intended duty cycle (Urban, Multi-Purpose, or Regional) for each of your vocational vehicle configurations based on the expected use of the vehicles.