The North American light duty vehicle fleet includes a substantial number of Flexible Fuel Vehicles (FFVs) which can operate on fuels containing up to 85% denatured ethanol. When these vehicles are operated in climatic regions with very cold winter temperatures, the portion of denatured ethanol in the fuel is normally reduced to well below 85% in order to improve fuel volatility. Both the U.S and Canadian standards for automotive spark ignition fuels include seasonal volatility requirements whose purpose includes the creation of fuel tank vapour space mixtures that are too rich to be flammable. However, there is limited information available about the actual temperature limits of flammability for real-world high ethanol (“E85”) and low ethanol (“E10”) blends, and the minimum vapour pressure values needed to achieve the desired temperature limits. In the present study field samples of fuel ethanol and gasoline were tested for vapour pressure, ethanol content, and hydrocarbon composition. Additional samples were prepared by blending denatured ethanol, winter Conventional Before Oxygenate Blending (CBOB) gasoline, and n-butane. For each sample, the temperature limit of flammability (the temperature at and below which flammable mixtures were formed) was measured using a specialized test method in which small rectangular combustion chambers were refrigerated while filled to 5% volume with the test fuel (intended to simulate a vehicle fuel tank at it’s minimum fill level). Ignition was then attempted using a high energy spark and flammability was determined based upon the level of pressure rise from combustion. The paper compares the measured flammability limits with the ambient temperatures of the field sampling locations, documents the relationship between vapour space flammability and vapour pressure, and discusses the requirements for achieving gasoline-like flammability performance at high ethanol levels.