One might imagine that the mechanism of helicases would relate to the number of base pairs that are unwound for each ATP that is hydrolysed. Recent studies, however, suggest the situation can be more complicated than this.
Wigley, D. B. (2000, June 1). DNA helicases: One small step for PcrA, one giant leap for RecBC? Current Biology. Current Biology Ltd. https://doi.org/10.1016/S0960-9822(00)00529-7