Just look at it this way:
The C-10 (now C-11) and C-12(now C-13) are based on the same architecture as the 3176 of days past. Our Peterbilt 377 feed truck has a C-10 in it, and it is most definitely a Class 8 truck.
The C-7, on the other hand, is based on the same architecture as the 3126 of a few years ago. GM put a shitload of these in TopKicks, Kodiaks, and maybe some other trucks for quite a while, making it a real pet of the MD set.
OK, class, CAT Diesel 101 is hereby DISMISSED!