The wind chill index is meaningless when it comes to starting your car. A human body is heated internally by the consumption of calories (food). That heat is circulated via blood streams. On a cold day, any wind whisks away some of that heat, forcing the cardiovascular system to work harder. At some point, the body knows to circulate less heat to the extremities in order to keep the internal parts warm so the body can survive. Fingers, toes, ears and other extremities are at risk of freezing.
This is the result of wind chill and has nothing in common with a car not starting. High wind can whisk away heat from an engine just turned off, so a car will cool down faster in the wind. But left out all night, the car will cool to the air temperature. A car in a garage is more likely to start because it's warmer inside. Even an unattached, unheated garage is warmed by heat rising up from the earth.