Low temperature operation of diesel vehicles can be improved either by diluting the fuel with low-wax diluents or by treating the fuel with flow improvers. The objective of this study was to determine the effect of the combined use of flow improvers and diluents. Laboratory experiments were conducted using three low-wax diluents - a No. 1-D diesel fuel, a tar sands-derived fuel, and an unleaded gasoline - in various No. 2-D diesel fuels. All three diluents increased the responsiveness of the No. 2-D fuels to flow improver treatment, and the combined use of diluents and flow improvers synergistically reduced the filter plugging temperature. For example, these reductions ranged from 6 to 13°C for fuels containing 25% No. 1-D fuel and a flow improver, whereas the use of flow improver alone was ineffective in most of the fuels, and dilution alone caused reductions of only 3 to 5°C. Vehicle tests confirmed the synergistic effect of dilution and flow improver use.