Kelley Blue Book’s Senior Managing Editor Matt DeLorenzo writes This Week in Car Buying, a weekly column on KBB.com. In this week’s article, Matt highlights the venerable Ford Mustang, which turned 55 at the New York Auto Show April 17. The company celebrated the milestone in two ways, boasting about the model’s sales leadership and introducing a new 2.3-liter High Performance Package that wrings 330 horsepower out of a 4-cylinder engine. On the luxury side, Audi announced that is has expanded its subscription program to include more affordable cars in the program. Car Buyer Journey, a new Cox Automotive Study on car buying, shows that more shoppers are becoming frustrated by higher vehicle prices and are looking more toward the used market to fill their transportation needs. And, keeping with that them, stocks of new vehicles continue to climb, reaching a total of nearly 4.2 million, raising the days’ supply figure for the industry to 70. Days’ supply is the number of days it would take to sell all the cars in inventory at the current sales rate.