IMP electron data are studied with the aim of determining a closure relation describing heat transport in the solar wind. We have compared a simple model of the relative drift speed between core-electron and proton populations, ΔVc, with measured values. In this model, core electron acceleration responds to the superposition of two opposing forces. The first results from proton-electron Coulomb friction with is characterized by the slowing down time &tgr;cp and tends to accelerate the core-electron population up to the proton bulk speed. Wave-electron collisions supplement this friction thereby increasing the core-electron acceleration. The second force results from the macroscopic interplanetary electric and magnetic fields and tends to reduce the core speed to zero with a time constant, &tgr;&sgr;, equal to the bounce preiod of a typical bound electron. At equilibrium, the two opposing forces balance yielding a relation between ΔVc and the solar wind bulk speed in a corotating reference frame, Usw:ΔVc=Usw/[1+&bgr;&tgr;&sgr;/&tgr;@qL cp>. The model predictions are found to agree best with measured values if &bgr;?4.5. A model for the heat flux, Q=&ggr;NckTcUsw/ [1+&bgr;&tgr;&sgr;/&tgr;cp>, with &ggr;=10.7 and &bgr;=4.5, implied by the above and previous results, also fits measurements at 1 AU. The results suggest that the model may be useful for closing the Boltzmann moment equations describing the solar wind expansion.