General pattern of reactivity

This table summarizes the general pattern of reactivity expected from various structural classes of alkyl halides in reactions with a representative range of nucleophiles. 

* poor nucleophiles are considered in this table in acidic conditions

* Methyl halides cannot eliminate as there are not appropriately placed protons

* Increasing branching favours elimination over substitution

* Good nucleophiles undergo substitution by SN2 unless the substrate is tertiary

* High temperatures favour elimination by gearing up the importance of entropy in the free energy of the reaction as ΔG=ΔH - TΔS, as you can find explained in another post. To increase temperature is a good way of ensuring E1 in ambiguous cases.