$C H_{3}-X>$ Primary $>$ Secondary $>$ Tertiary

However, if the primary alkyl halide or the nucleophile/base is sterically hindered the nucleophile will have difficulty to getting the back side of the $\alpha$ - carbon as a result of this, the elimination product will be predominant.

Here, $C H_{3} C H_{2} B r$ is the least hindered, hence it has the highest relative rate towards $S_{N^{2}}$ reaction.