Section 1037.231 - Powertrain families(a) If you choose to perform powertrain testing as specified in 40 CFR 1036.545, use good engineering judgment to divide your product line into powertrain families that are expected to have similar fuel consumptions and CO2 emission characteristics throughout the useful life. Your powertrain family is limited to a single model year.(b) Except as specified in paragraph (c) of this section, group powertrains in the same powertrain family if they share all the following attributes: (1) Engine family as specified in 40 CFR 1036.230 .(2) Shared vehicle service class grouping, as follows:(i) Light HDV or Medium HDV.(ii) Heavy HDV other than heavy-haul tractors.(iii) Heavy-haul tractors.(4) Type of clutch (e.g., wet or dry).(5) Presence and location of a fluid coupling such as a torque converter.(6) Gear configuration, as follows:(i) Planetary (e.g., simple, compound, meshed-planet, stepped-planet, multi-stage).(ii) Countershaft (e.g., single, double, triple).(iii) Continuously variable (e.g., pulley, magnetic, toroidal).(7) Number of available forward gears, and transmission gear ratio for each available forward gear, if applicable. Count forward gears as being available only if the vehicle has the hardware and software to allow operation in those gears.(8) Transmission oil sump configuration (e.g., conventional or dry).(9) The power transfer configuration of any hybrid technology (e.g., series or parallel).(10) The energy storage device and capacity of any hybrid technology (e.g., 10 MJ hydraulic accumulator, 10 kW·hr Lithium-ion battery pack, 10 MJ ultracapacitor bank).(11) The rated output of any hybrid mechanical power technology (e.g., 50 kW electric motor).(c) For powertrains that share all the attributes described in paragraph (b) of this section, divide them further into separate powertrain families based on common calibration attributes. Group powertrains in the same powertrain family to the extent that powertrain test results and corresponding emission levels are expected to be similar throughout the useful life.(d) You may subdivide a group of powertrains with shared attributes under paragraph (b) of this section into different powertrain families.(e) In unusual circumstances, you may group powertrains into the same powertrain family even if they do not have shared attributes under in paragraph (b) of this section if you show that their emission characteristics throughout the useful life will be similar.(f) If you include the axle when performing powertrain testing for the family, you must limit the family to include only those axles represented by the test results. You may include multiple axle ratios in the family if you test with the axle expected to produce the highest emission results. 81 FR 74048 , Oct. 25, 2016, as amended at 86 FR 34462 , June 29, 2021; 88 FR 4639 , Jan. 24, 2023; 89 FR 29777 , Apr. 22, 2024 81 FR 74048, 10/25/2016, as amended at 86 FR 34462, 6/29/2021; 88 FR 4639, 3/27/2023; 89 FR 29777, 6/21/2024