This is US only sales and the worst time of year to sell a new performance car too! I can see sales numbers going up in the spring when people who get harsh winters can start thinking about driving performance cars again.
So this begs the question, why aren't all Mustangs (and the competitors) 1/2 model years and released in the beginning of Q2 instead of Q4? Is it to even out the sales throughout the year? Limited production capacity?