40 CFR §1037.232

1. (a)If you choose to perform axle testing as specified in § 1037.560 or transmission testing as specified in § 1037.565, use good engineering judgment to divide your product line into axle or transmission families that are expected to have similar hardware, noting that efficiencies can differ across the members of a family. Note that, while there is no certification for axle and transmission families under this part, vehicle manufacturers may rely on axle and transmission test data to certify their vehicles.
2. (b)Except as specified in paragraph (d) of this section, group axles in the same axle family if they have the same number of drive axles and the same load rating.
3. (c)Except as specified in paragraph (d) of this section, group transmissions in the same transmission family if they share all the following attributes:
   1. (1)Number and type of clutches (wet or dry).
   2. (2)Presence and location of a fluid coupling such as a torque converter.
   3. (3)Gear configuration, as follows:
      1. (i)Planetary (e.g., simple, compound, meshed-planet, stepped-planet, multi-stage).
      2. (ii)Countershaft (e.g., single, double, triple).
      3. (iii)Continuously variable (e.g., pulley, magnetic, toroidal). Note that GEM does not accommodate efficiency testing for continuously variable transmissions.
   4. (4)Transmission oil sump configuration (conventional or dry).
4. (d)You may subdivide a group of axles or powertrains with shared attributes under paragraph (b) or (c) of this section into different families.