SN2: Neopentyl halides and methyl halides
Carey-Sundberg has some answers (vol. A, chapter 4).  The text is a bit advanced for you at the present stage, but you should give it a try.

It gives as reference JACS 1975, 97, 3694.  That paper has rate constants for nucleophilic substitution of compounds of type $\ce{RCH2Br}$, with $\ce{R=H, Me, Et, i-Pr, t-Bu }$ with $\ce{LiCl}$ in acetone at room temperature.

Rate constants in 105 s-1:
$\ce{R=H}$: 600, $\ce{R=Me }$: 9.9, $\ce{R=Et}$: 6.5, $\ce{R=i-Pr}$: 1.5, $\ce{R=t-Bu }$: 2.6*10-4

Nucleophilic substitution does take place in neopentyl halides, but at a rate of five orders of magnitudes less than in less hindered compounds.