The perkins variant of the class 158 is virtually identical the the Cummins variant currently in game, aside from sounding slightly different, while cummins 158s are louder and more screamy, perkins 158s are a lot more tame (and in my opinion better sounding). Adding this variant would create a different feel when driving the 158, with 2 different sounding options to chose from. Some examples of the perkins sounds:
