The dependence on the ratio of the electric field to the neutral particle number density of reaction rate constants measured in drift tubes can be approximated asymptotically near zero field by the logarithm of the relative speed distribution of an ion and a neutral molecule, evaluated at the threshold velocity. This approximation adequately represents the field dependence of the rate constant into the region where experimental measurements of the rate constant can be obtained, allows a quick determination of the threshold velocity from rate constant measurements, and gives a consistency check on proposed ion‐molecule relative speed distributions.