The evolution of engine-generated chemiions in a vehicle tailpipe or in a sampling transfer line is studied and the key parameters controlling chemiion concentration at the exit plane are investigated. We find that the observed higher number concentration but smaller size of nuclei mode particles associated with shorter transfer line residence time, enhanced nanoparticle emission associated with reduced soot emission, and increase in the number concentration but decrease in the size of nuclei mode particles associated with increasing traffic speed, can all be consistently explained by the reduced loss of ions in the transfer line or tailpipe. This provides further evidence that engine-generated chemiions play a key role in the formation of the nanoparticles in motor vehicle exhaust.