My book says, and I understand, that the Sn1 mechanism is much faster for tertiary alkyl halides than primary. And the opposite for Sn2. This all makes sense. What I don't get is the conclusion it comes to next: hence, we predict that the primary alkyl halides will react primarily via Sn2, and the tertiary via Sn1. The reason I get confused here is that nothing was said about the comparative rates for a primary alkyl halide via the Sn1 or Sn2 ( they just compared primary and tertiary for SN1 and SN2). In my mind, which mechanism dominates for primary or tertiary should be dependent on the relative rates for that compound; not rates compared between primary and tertiary.
Is my argument sound? And if so, is there some literature you know of that explains this in such a manner?