A joint program was undertaken to evaluate the use of Middle Distillate Flow Improvers (MDFIs) in diesel fuel to ambient temperatures as low as -40°C. The objectives of the program were to (i) study MDFI effectiveness at preventing fuel filter blockage due to excessive wax formation at temperatures below -30°C, and (ii) determine the effectiveness of Low Temperature Filterability Test (LTFT) laboratory procedure in protecting vehicles these extremely low temperatures. A total of seven fuels were blended (including 4 treated with MDFI additive) and subsequently tested in three heavy duty trucks in an all weather climate controlled chassis dynamometer facility. Overnight soak temperatures were as low as -40°C. Two of the trucks were equipped with an engine that was known to be critical for fuel filter plugging due to excessive wax formation. A total of 14 valid vehicle tests were run over a six-day period. The results obtained from this trial indicate that at these extremely low ambient temperatures (i) MDFIs can be used effectively and (ii) use of the LTFT will protect vehicle operability.